A financial services client running 400 outbound calls per day was paying £0.022 per minute on Twilio's standard carrier pool. Their voice agents handled financial advice calls averaging 10 minutes each. In week eight of our engagement, we moved them onto a direct SIP trunk with Gamma. Per-minute cost: £0.009. At 400 daily calls running 10 minutes apiece, that £0.013/min rate delta adds up to £19,000 per year in savings. The compliance team also needed SIPREC-format recordings routed to a UK-region S3 bucket with specific metadata headers for FCA record-keeping — something the shared pool couldn't support without custom webhook middleware that added 200ms of latency to every call. Both had the same root: shared carrier pools trade control for convenience, and past 150 daily calls most teams have outgrown that trade-off.
SIP trunking for UK voice agents: shared pool vs direct carrier cost breakdown
The arithmetic is straightforward once you have real numbers. Here's what the comparison looks like at production UK call volumes:
| Twilio Shared Pool | Direct SIP Trunk (Gamma / Simwood) | |
|---|---|---|
| Per-minute cost (UK outbound) | £0.018–£0.022 | £0.007–£0.010 |
| Number ownership | Twilio-hosted | Ported to your account |
| SIPREC recording | Not native | Native on wholesale tier |
| Failover control | Platform-managed | You configure it |
| Provisioning time | Minutes | 2–5 business days |
| Monthly minimum | None | £50–£150 |
At 100 daily calls with a 90-second average — 4,500 minutes per month — the £0.013/min delta saves roughly £58/month. That barely covers a basic carrier contract. At 300 daily calls (13,500 minutes/month), the same delta saves £175/month. The monthly minimum stops being relevant above roughly 200 daily calls.
Ofcom's fixed call termination rate reviews document UK wholesale termination costs falling steadily — some analysts argue this will erode the direct trunk cost advantage over the next two to three years. At current rate levels, that convergence requires a further 50% compression of the spread; teams running regulated outbound today cannot wait on it.
The less-discussed cost is number reputation: on a shared pool your outbound CLI comes from IP ranges shared with every other customer. On a direct trunk, your IP block is yours. See our phone number warmup and spam recovery guide for the mechanics.
How SIP trunking works in a voice agent stack: the protocol layer most teams skip
SIP is a signalling protocol — it sets up, manages, and tears down calls. RTP carries the actual audio. These are separate channels, and the separation matters for how you architect latency and recording.
Voice Agent Application
│
├── SIP Signalling (TCP/TLS, port 5060/5061)
│ └── INVITE, ACK, BYE, REFER (for transfers)
│
└── RTP Media Stream (UDP, ports 10000–20000)
└── Bidirectional audio, 20ms packetisation
SIP Trunk (Gamma / Simwood)
├── Receives SIP INVITE from your application
├── Routes to PSTN
└── SIPREC fork → your Session Recording Server
Your SIP trunk provider does not process your audio — they route it. A direct trunk with a UK Tier 1 carrier reduces the media path to a single hop between your application server and the PSTN, typically saving 20–40ms of round-trip latency versus Twilio's platform.
For high-volume outbound agents, use SIP REGISTER mode rather than INVITE mode — it maintains a persistent session, removing per-call TLS handshake overhead that accumulates at 20+ concurrent calls.
UK SIP carrier selection: BT Wholesale, Gamma, Vonage, and Simwood compared
| Carrier | Per-minute (outbound) | SIPREC | Min. contract | BYOC support |
|---|---|---|---|---|
| Gamma | £0.007–£0.010 | Native | ~£75/month | Yes |
| Simwood | £0.006–£0.009 | Native | ~£50/month | Yes |
| BT Wholesale | £0.008–£0.012 | On request | ~£150/month | Via partner |
| Vonage Business | £0.009–£0.015 | Enterprise tier only | ~£100/month | Via API |
Gamma is our default for UK SMEs new to direct SIP. Their wholesale product has solid geographic coverage, sub-50ms connection times to the London PoP, and native SIPREC support on the wholesale tier. Onboarding takes 2–3 business days once you have a signed order form.
Simwood is the better choice when per-minute rates are the primary constraint. They are a UK Tier 1 interconnect carrier — not a reseller — so your traffic takes fewer hops to the PSTN. Their developer documentation is genuinely useful, and they publish their peering agreements publicly. The trade-off: support is more technical and less guided than Gamma's.
BT Wholesale fits for ISDN estate migrations and specific geographic routing requirements. Their commercial process requires a reseller partner in most cases; onboarding runs 2–4 weeks.
Vonage is the weakest option for voice agent workloads. SIPREC is restricted to enterprise contract tiers, and per-minute pricing carries platform fees that Gamma and Simwood do not charge.
Number porting for outbound: what UK carriers require and how long it takes
UK number porting is governed by Ofcom's porting rules, which require the losing carrier to complete a port within 5 working days of receiving a valid Letter of Authorisation (LOA). In practice, timelines split into two categories:
Simple ports (single numbers, no CPS configuration): 5–10 business days end-to-end.
Complex ports (number ranges of 100+, numbers with active Carrier Pre-Select records, or multi-site ISDN migrations): 15–30 business days.
Porting away from Twilio: Twilio is registered as an end-user carrier in the UK. Raise the request through Twilio's support portal with your UK account address and the gaining carrier's LOA. Budget 10–15 business days — their process runs through standard support queues.
The rule we apply: never enter a porting window without a working standby number on the destination carrier. Port dates slip 48 hours regularly. Pre-configure a secondary campaign number on the new trunk and cut the primary over once the port confirms. Our FCA and PECR voice remarketing guide covers what sits at the application layer regardless of which carrier you use.
SIPREC-compliant recording over SIP: the compliance path FCA-regulated firms need
The FCA's SYSC 10A rules on telephone recording apply to any regulated firm communicating about financial instruments. The requirements that matter for voice agent deployments:
- Recordings retained for a minimum of 5 years (7 years for MiFID II scope activity)
- Stored in a tamper-evident format with metadata identifying the caller, agent ID, and timestamp
SIPREC handles this at the carrier layer — no application-side recording logic required. The carrier forks the call audio to your Session Recording Server before it reaches the PSTN. Here is a working Gamma trunk SIPREC configuration:
# SIP trunk SIPREC configuration — Gamma wholesale (example values)
siprec:
enabled: true
transport: TLS
srs_uri: "sip:[email protected]:5061"
metadata_headers:
X-Recording-Direction: outbound
X-Agent-ID: "{{ agent_id }}" # injected per-call by your application
X-Campaign-ID: "{{ campaign_id }}"
X-Regulated-Activity: "mortgage-advice"
storage:
provider: s3-compatible
region: eu-west-2 # UK data residency
bucket: fca-regulated-recordings
encryption: AES-256
retention_years: 7
Webhook-based recording joins the call as a participant, adding call legs and latency. SIPREC forks before the audio reaches your application — no latency hit, no middleware failure mode. Our voice AI document analysis case study covers the metadata tagging layer we built on top of a similar SIPREC implementation for an FCA-regulated deployment.
Failover design: what happens when your SIP trunk drops mid-campaign
Design trunk failure into the architecture from day one. The primary/secondary configuration we run in production:
{
"sip_trunks": [
{
"name": "gamma-uk-primary",
"hostname": "sip.gamma.co.uk",
"transport": "TLS",
"priority": 1,
"credentials": { "username": "qa-outbound-01", "realm": "sip.gamma.co.uk" }
},
{
"name": "simwood-uk-failover",
"hostname": "pbx.simwood.com",
"transport": "TLS",
"priority": 2
}
],
"health_check": {
"method": "OPTIONS",
"interval_seconds": 30,
"failure_threshold": 3,
"recovery_threshold": 2
}
}
The SIP gateway sends OPTIONS pings every 30 seconds. Three consecutive failures trigger automatic failover; new call attempts route through Simwood while in-flight calls on Gamma complete normally. Two clean pings restore the primary. OPTIONS checks miss carrier-side audio degradation — detecting that requires active MOS monitoring on the RTP stream, covered in our voice agent monitoring and alerting stack post.
Twilio Elastic SIP Trunking vs direct carrier: when each wins
Twilio's Elastic SIP Trunking is a distinct product from their shared carrier pool, sitting between the two in cost and control.
Use Twilio SIP Trunking when: - You're already inside the Twilio ecosystem (Programmable Voice, Studio, Flex) - Your volume is 150–400 daily calls — above the shared pool crossover, below the threshold where direct carrier negotiation clearly wins - Fast setup matters and you don't want to manage a carrier relationship directly - SIPREC compliance is not a requirement
Use a direct carrier (Gamma/Simwood) when: - Volume exceeds 400 daily calls — direct rates typically undercut Twilio SIP by 20–30% - FCA or SIPREC compliance is required - You need numbers ported to a carrier independent of your voice platform - You want failover control across genuinely different Tier 1 interconnect paths
Twilio SIP is a reasonable staging point for teams moving off the shared pool. It is not the terminal state for a regulated outbound operation at scale. For the full platform comparison, see our Twilio vs Retell vs VAPI analysis.
What changed in 2025–2026: native SIP REFER support in Retell and VAPI
The most significant operational change for UK voice agent deployments in this period was native SIP REFER support landing in both Retell and VAPI. Previously, warm transfers over a direct SIP trunk required the application layer to orchestrate the handoff — end the first call leg, dial the transfer target, hold the caller while the second leg connected — producing 2–4 seconds of dead audio that callers on financial services calls routinely mistake for a disconnection.
SIP REFER hands the transfer to the signalling layer. The carrier handles the handoff without your application acting as intermediary, eliminating the dead audio window and the race conditions that arose when the second leg connected before the first was fully released. Retell shipped REFER support in their v2.1 BYOC release in Q4 2025; VAPI reached general availability in April 2026. If your BYOC configuration was built before late 2025, check whether you're still running two-leg transfers.
Frequently asked questions
At what daily call volume does SIP trunking become cheaper than Twilio's carrier pool in the UK?
The crossover sits at 150–200 outbound calls per day. At that volume, per-minute savings on a direct trunk (£0.007–£0.010 vs £0.018–£0.022 on shared pools) exceed typical carrier monthly minimums (£50–£150). Below 150 daily calls, the shared pool wins: no minimum, faster setup. Above 400 daily calls, the annual saving regularly exceeds £15,000.
How long does UK number porting take, and can I port numbers from Twilio to a SIP carrier?
Standard UK porting takes 5–10 business days after both carriers accept a valid Letter of Authorisation (LOA). Complex ports — ranges over 100, or numbers with active CPS configurations — run 15–30 business days. You can port away from Twilio via their support portal; budget 10–15 business days. Always pre-configure a standby number on the destination carrier before the porting window opens — port dates slip 48 hours without warning.
Do I need a separate SIP carrier if I'm already using Retell or VAPI for voice agents?
Usually yes if you're above 100 daily calls or have compliance requirements. Both Retell and VAPI default to shared carrier pools. Both support BYOC via SIP trunk configuration — Retell through BYOC trunk settings, VAPI through their SIP trunk endpoint. Bringing your own trunk gives you direct carrier rates, SIPREC recording paths, carrier-independent number portability, and failover you control.
What does SIPREC compliance require, and which UK SIP carriers support it natively?
SIPREC requires the carrier to fork call audio to a Session Recording Server you control before it reaches the PSTN. For FCA-regulated firms: recordings must be tamper-evident, stored in a UK data region, retained for five years minimum (seven for MiFID II), with caller, agent ID, and timestamp metadata. Gamma and Simwood support SIPREC natively on wholesale trunks. Vonage restricts it to enterprise contract tiers.
Good / Bad / Ugly: three trunking configurations and the failure each one caught
Good: Primary/secondary trunks with OPTIONS health check
A UK mortgage firm running 300 daily outbound calls. Primary on Gamma (London PoP), failover on Simwood (different Tier 1 path). When Gamma had a 40-minute routing issue one Tuesday morning, Simwood absorbed the traffic within 90 seconds of the third failed OPTIONS ping. The campaign continued without interruption. The client saw a brief dip in CDR logs and nothing else. Cost of the standby Simwood contract: £50/month.
Bad: Single trunk, no SIPREC, webhook-based recording
A property management client migrated to a direct SIP trunk for cost savings but kept their existing webhook recording architecture. Every call fired a webhook to a Lambda function that joined the call as a recording participant — adding 180–220ms to call setup time and causing DTMF timeout errors during payment verification flows. Migrating to native SIPREC took two days. It should have been part of the initial trunk migration.
Ugly: Porting 40 numbers without a standby bank
A client ported their entire outbound number range from Twilio to Gamma without pre-configuring standby numbers on the destination. The port date slipped 48 hours — standard in complex ports. During that window, numbers were in a liminal state: not fully active on either carrier. Three campaigns paused; recovery required emergency Gamma provisioning and manual campaign reconfiguration at 7am on a Wednesday. The rule: never enter a port window without a working alternative number range on the destination carrier, and always build the standby before the window opens.
We scope SIP trunk configurations for UK voice agent deployments as part of our 21-day engagement. If you're running more than 150 daily calls on a shared pool, book a 30-minute review and we'll work through the carrier options, porting timeline, and SIPREC requirements — you'll leave with a clear carrier recommendation, a migration checklist, and a failover configuration suited to your actual call volume.