Identify the exact product and order
Spitfire presents ordinary SIP trunks, connectivity, number supply, gateways and Microsoft Teams Direct Routing as related but distinct capabilities. The Teams offer includes a provider-managed SBC; that does not establish the edge design for an ordinary PBX trunk. Record the legal account, ordered product, numbers, ports, channels, PBX or SBC, access circuit, support level and current technical pack before choosing a template.
A product-page claim or interoperability list is not proof that an option is included, presently available or compatible with a particular software build. Preserve the dated order, provisioning sheet and observed portal state separately.
Documented service facts
Reviewed 2026-10-11. Compare SIP trunk services →
SIP authentication
Not verifiedSpitfire describes connecting an existing phone system to its SIP network, but the reviewed public documents do not define one registration, digest-credential or source-IP authentication method for every order. Obtain the current provisioning sheet and record direction, SIP identity, authorization identity, source address or FQDN, challenge and observed response without treating the methods as interchangeable.
SIP registration
Not verifiedThe reviewed public pages do not establish whether REGISTER is required, optional or absent on the selected trunk, nor do they publish a registrar, proxy, Contact policy, expiry or refresh interval. Bind those values to the account-specific technical pack and observed responses.
Inbound delivery and number format
Not verifiedSpitfire advertises UK and international numbers and porting, but the reviewed pages do not publish a universal Request-URI, number format, source range, destination port or identity-header contract. Retain the assigned number, port result, delivery target and an observed SIP message for every route.
Inbound resilience
DocumentedSpitfire documents geographically diverse SIP nodes, outbound resiliency and options to divert calls to a predetermined alternate number or combine the service with an independent backup path. These descriptions do not establish detection or restoration timing, active-call survival, capacity or identity behavior for the selected order. Separate SIP-node, number-diversion and access-circuit tests.
Number and regional availability
Not verifiedSpitfire says it can supply numbers from UK dialling codes and international locations and port existing numbers. That does not prove that every country, rate centre, number type, address or existing number is available or portable on a given date. Obtain a dated inventory or port result for the exact legal entity and service address.
Audio codecs and DTMF
Not verifiedThe reviewed public product, terms and service documents do not establish a codec list, packetization interval, DTMF method, RTP address range or media-latching behavior for the ordered trunk. Obtain the current technical pack and preserve the actual SDP offer and answer, telephone-event mapping, RTP endpoints and decoded result.
Fax and T.38
Not verifiedThe reviewed public documents do not establish T.38 support, audio-fax fallback, direction, gateway interoperability or document limits for the SIP trunk. Qualify fax separately using a provider-approved representative document set, and do not infer support from ordinary voice completion.
Signaling and media encryption
Not verifiedSpitfire markets dedicated connectivity and network controls, but the reviewed public material does not establish TLS, certificate, SRTP, keying or customer-facing IP-filter requirements for the selected trunk. Treat access isolation, SIP authorization, signaling confidentiality, media confidentiality and fraud controls as separate controls.
Concurrent calls and call-start rate
Not verifiedThe product material says channel quantities can be changed, but it does not publish the selected account entitlement, bursting rule or calls-per-second limit. Do not convert channel count, number inventory or scalability language into a CPS allowance. Obtain dated limits and run only a separately approved bounded load test.
Emergency calling
DocumentedSpitfire says customers whose SIP service provides emergency-call access must register a fixed service location before activation and recommends updating location information when service is used from multiple locations. Confirm that emergency access is included, the registered address and callback number, nomadic behavior, outage alternative and provider-approved validation process for each site; do not place an uncoordinated emergency call.
Pricing and contract
Not verifiedThe product page advertises SIP trunking from £4 per month, but that headline does not establish the complete current price for a selected trunk, channels, numbers, ports, destinations, connectivity, support, taxes or renewal. The telephony terms say charges follow the notified tariff and may change. Obtain the current order form, tariff, minimum period, renewal and cancellation terms in writing.
Administration and support
Not verifiedSpitfire publishes SIP-service response and clearance targets and 24-hour fault-case handling in its public SLA, while also warning that component faults can sit outside SIP-trunk clearance targets. A public SLA does not prove incorporation into the selected order or every maintenance, escalation and service-credit condition. Bind operations to the signed contract and exact service level.
SMS and MMS
Not verifiedThe reviewed SIP-trunk documents do not establish SMS or MMS eligibility, sender registration, API scope, throughput, retention or price for a trunk number. Qualify messaging as a separate number and commercial service rather than inferring it from voice routing or number supply.
Bind signaling to the provisioned trunk
The reviewed public documents do not define a universal Spitfire authentication or registration model. Determine whether each call direction uses registration, digest credentials, an approved source address or another provisioned method.
Retain registrar or proxy, DNS result, transport, SIP and authorization identities, source address or FQDN, Contact, expiry, challenge and response. A portal status, REGISTER response or OPTIONS reply does not by itself prove inbound and outbound completion.
Separate number supply, porting and identity
Spitfire advertises number supply across UK dialling codes, international locations and porting agreements. Confirm the exact number, service address, portability result and go-live window rather than treating broad coverage as current inventory.
For inbound calls, retain source, destination, Request-URI, asserted identity, PBX match and final extension. For outbound calls, retain the authorized identity, presented caller number, privacy behavior and carrier response. Do not infer caller-ID authorization from number ownership or inbound delivery.
Negotiate media and security from current evidence
The reviewed public material does not publish a universal codec, packetization, DTMF, RTP, T.38, TLS or SRTP contract. Obtain the current technical pack and configure only methods supported by both the selected trunk and PBX.
Preserve offer and answer SDP, telephone-event mapping, RTP endpoints, decoded result, signaling transport, certificate and media-protection profile where applicable. Dedicated connectivity or provider-managed routing does not by itself prove customer-facing SIP encryption, media encryption or fraud prevention.
Test each resilience layer independently
Spitfire describes geographically diverse SIP core nodes, number diversion, independent backup access and, for some access designs, manual or automatic circuit failover. These are different layers. Map each trigger, alternate path, required action, capacity, caller identity, detection, restoration and return-to-primary behavior.
Spitfire advises monthly testing of backup solutions and notes that configurations differ. Its documented access-circuit examples, including possible multi-minute switchovers, must not be presented as SIP-trunk failover timing. Prove new-call routing and recovery with an approved exercise; do not claim that established calls survive unless observed.
Accept emergency, operational and commercial limits
Spitfire requires location registration for SIP services that provide emergency access at a fixed location and recommends updates for multi-location use. Confirm emergency access, address, callback number, nomadic behavior, power and connectivity dependencies, outage alternative and provider-approved validation method; do not place an uncoordinated emergency call.
Reconcile the signed order with the current tariff, telephony terms and applicable SLA. Public prices and targets do not prove the selected contract. Record explicit channel and CPS limits, support and escalation obligations, service credits and a bounded acceptance matrix for signaling, inbound, outbound, identity, audio, DTMF, protection, failover and rollback.
Sources & applicability
Primary references for the technical details above. Operational examples and planning checklists are VoIP.info editorial guidance.
- Spitfire SIP Trunks ↗
- Spitfire SIP Trunks product sheet ↗
- Spitfire emergency location information ↗
- Spitfire telephony terms of service, version 5 ↗
- Spitfire service level agreement ↗
- Spitfire backup IP circuits advice ↗
- Spitfire Direct Routing for Microsoft Teams ↗
Examples require adaptation to your topology. No live PBX or hardware testing is claimed.