Start with the ordered service version
BT's September 2025 terms separate full PSTN or ISDN replacement from outbound-only service. They also allow the trunk to stand alone or act as access to another BT or third-party service. Record the exact country, Site, service version, number scope, destination service and Order before transferring any capability into a design.
Full replacement may provide inbound, outbound and emergency calling; the outbound version does not provide emergency calling. A product-family label, neighboring cloud service or country where BT operates does not establish which version is available or ordered for one Site.
Documented service facts
Reviewed 2026-10-07. Compare SIP trunk services →
SIP authentication
Not verifiedThe reviewed public service schedules define access and management boundaries but do not publish one universal customer authentication profile. Preserve the ordered service version, Site, interconnect, source addresses, credentials or certificates, SBC policy and observed authorization result; do not infer registration or IP authentication from the product name.
SIP registration
Not verifiedThe public contract set does not establish REGISTER as the universal interconnect model or publish one registrar, realm, Contact policy or expiry. Obtain the order and implementation worksheet, then retain the actual connection method and signaling evidence. A commissioned access circuit or accepted service does not by itself prove a PBX registration.
Inbound delivery
DocumentedThe September 2025 terms separate full PSTN replacement, which may include inbound and outbound calling, from outbound-only service. Part B makes geographic numbers and number presentation order-dependent and allows public or private dial plans. Preserve the Site's ordered service version, number inventory, port state, delivered identity and selected PBX route.
Routing and failover
DocumentedPart B describes optional network overflow through up to three defined destinations, while the product page discusses centralized service and resilience. Overflow policy, access diversity, alternate Site, PBX failover, new-call recovery and established-call survival remain separate. Record the ordered option and run an approved failure exercise before claiming resilience.
Number and regional availability
DocumentedBT's product page describes broad country reach, but Part A says the available service version depends on country and Site and number portability is subject to local regulation, technical feasibility and carrier arrangements. Product reach is not serviceability, number inventory, port acceptance or activation for one address; obtain the current order and country exhibit.
Audio codecs
Not verifiedThe product page says multiple codecs are supported, but the reviewed public schedules do not publish one codec, packetization, DTMF, transcoding or media-anchoring profile for every country and access model. Preserve both SDP bodies, the selected codec and DTMF method, media endpoints and decoded audio per direction for the ordered Site.
T.38 fax
Not verifiedThe reviewed current first-party product and service schedules do not establish T.38, G.711 pass-through, redundancy, page limits or gateway behavior for every Site. Treat fax as a separate design and acceptance path using the exact country, access, PBX, gateway and order rather than transferring a claim from another BT voice product.
Signaling and media encryption
DocumentedPart B says a BT Internet access order selects encrypted signaling with either encrypted or unencrypted voice; third-party and partner access require an agreed encryption configuration. These are ordered choices, not proof of TLS version, certificate policy, SRTP profile or effective protection on either PBX-facing or carrier-facing legs. Retain the agreement and observed negotiation.
Concurrent calls and call starts
Not verifiedThe product page reports aggregate platform scale, not one customer's purchased channels or call-start controls. Preserve ordered channel or call-admission limits, trunk groups, Site distribution, burst policy, PBX and SBC ceilings and access bandwidth separately. Do not convert platform totals, an accepted test or an SLA into account capacity.
Emergency calling
DocumentedFull-PSTN replacement and outbound-only service have different emergency boundaries: Part A says outbound-only service cannot call emergency services, while full replacement is subject to country, number and location rules. Maintain each Site and number location, user notices, change ownership and an alternative calling method; use only provider-approved validation.
Pricing model
Not verifiedThe public terms place charges, optional features, reconfiguration, commitments, invoicing and termination in the Order and also describe traffic-profile and fair-use conditions. No universal current rate card or account quote was established. Obtain dated country, Site, access, channel, number, usage, support, tax and term records before comparison.
Administration and support
DocumentedBT publishes a self-service administration and reporting portal and contract maintenance, incident and acceptance processes. Part B limits management responsibility by access model and says the customer performs acceptance tests in the defined period. Preserve named roles, contacts, maintenance notices, management boundary, acceptance record and the applicable service schedule.
SMS and MMS
Not verifiedThe reviewed One Voice Global SIP Trunking product and schedules do not establish SMS or MMS entitlement on service numbers, sender registration, API access, throughput, webhook security, retention or price. Keep messaging and adjacent BT collaboration or contact-center products outside this record unless the exact order includes them.
Keep access, encryption and responsibility aligned
Part B separates BT Managed Access, third-party Internet and a partner-delivered GSIP Lite path. The service-management boundary changes with that choice. A BT Internet access order also distinguishes encrypted signaling with encrypted voice from encrypted signaling with unencrypted voice, while other access models require an agreed encryption configuration.
Record the router or Internet handoff, customer equipment owner, signaling and media paths, certificate and SRTP policy, and the observed negotiation. Managed access is not proof of encrypted media, and an ordered encryption option is not proof that both legs of a specific call used it.
Treat dial plans, identities and overflow as ordered policy
Part B permits public E.164 or fixed-length private dial plans and describes optional overflow through up to three destinations. Part A separately governs asserted, diversion and presentation identities and notes that delivery across third-party networks is not guaranteed.
Preserve the ordered number inventory, Site, dial-plan normalization, asserted identity, presentation policy, overflow sequence and every observed result. Overflow is a new-call routing control, not evidence of access diversity, PBX recovery or survival of established calls.
Measure Site capacity instead of using platform scale
BT publishes global platform and country-scale figures, but those are not one customer's channel entitlement, call-start rate or accepted load. Capture the ordered Site and trunk-group capacity, call-admission limits, access bandwidth, PBX and SBC ceilings and any traffic-profile controls independently.
Part A's service levels apply only inside their stated boundary and conditions. A target for network performance, service acceptance or aggregate platform size does not prove customer-site availability, third-party Internet quality, off-net completion or a tested failure outcome.
Bind emergency calling, price and acceptance to current records
Full-replacement emergency calling depends on the country, number and registered Site or location. Outbound-only service requires another compliant means. Maintain every number-to-location record, user notice, change owner and alternative calling path, and follow the provider's approved validation process.
Charges, optional features, reconfiguration, term and traffic conditions live in the Order and applicable schedules. Part B also defines a bounded acceptance period and management handoff. Do not represent a public product description as serviceability, accepted number inventory, current price, installed compatibility or laboratory validation. No account, Site, trunk, media, overflow, emergency workflow, availability or price was tested for this guide.
Sources & applicability
Primary references for the technical details above. Operational examples and planning checklists are VoIP.info editorial guidance.
- BT One Voice Global SIP Trunking product ↗
- BT Global Business service-schedule index ↗
- BT One Voice Global SIP Trunking Part A, September 2025 ↗
- BT One Voice Global SIP Trunking Part B, March 2025 ↗
- BT outbound-voice country obligations, July 2025 ↗
Examples require adaptation to your topology. No live PBX or hardware testing is claimed.