An IP SIP trunk connects a compatible phone system to a telephone service over an IP network, allowing incoming and outgoing calls without a traditional ISDN circuit. Choose channel capacity based on the number of simultaneous calls your business needs, not simply the number of telephone numbers.
What an IP SIP trunk actually does
Session Initiation Protocol (SIP) establishes, changes and ends voice sessions; RTP normally carries the audio. The SIP trunk links a compatible PBX or session border controller to the provider gateway.
An assigned DDI is the number customers dial. The SIP trunk is the connection handling calls. Multiple DDIs can use the same PBX and channel pool if the service is configured accordingly.
SIP trunks, channels and concurrent calls
At UKDDI the commercial model is one SIP trunk per concurrent call. A customer expecting four simultaneous calls needs capacity for at least four calls, irrespective of whether they have two DDIs or two hundred.
Inbound and outbound call direction does not automatically create extra paid capacity. Any separately published technical ceiling is not a promise of included paid concurrency. Check the exact plan and provisioning with UKDDI.
- Estimate peak concurrent inbound and outbound call legs.
- Include forwarded calls when evaluating capacity and billable legs.
- Ask which UK and international call destinations are included.
- Confirm the 2,000-minute fair-usage allowance per trunk on applicable unlimited plans.
- Autodiallers above 30 outbound calls per hour per trunk are not supported under UKDDI policy.
Technical checks before connecting your PBX
Confirm IP authentication or SIP registration, the provider’s signalling and media IPs, codec support, outbound CLI authorisation, NAT and firewall rules, SIP ALG behaviour and resilience design.
Test both inbound DDIs and outbound calls, caller ID, failure handling and the agreed emergency-calling arrangement. Do not presume a SIP trunk alone supplies emergency location information.
Only advertise capacities and resilience arrangements that the carrier and PBX configuration enforce. The current switch sandbox is not a live DDI provisioning API.
Questions to ask when comparing SIP trunk prices
Compare the monthly price per channel, included destination groups, call charges beyond the allowance, setup or number charges, minimum term, number porting and support availability. A low per-channel price may not represent the total cost of high-volume mobile or international calling.
UKDDI can quote channels and numbers separately and provide custom tariffs where supported. Request the details in writing so the accepted quote matches future invoices.
SIP trunk and DDI quotation
Request SIP channel capacity, DDI quantities, international bundles and tailored business pricing on one quotation.
Discuss a SIP trunk