Anchor the design to the ordered static peer and access path
Colt's external guide describes VoIP Access as a static SIP trunk between fixed Colt SBC and customer IP addresses and says registration mode is unsupported. Start with the implementation record: ordered service, country, access type, customer source addresses, assigned SBC peers, ports, transport, DNS treatment, number ranges and change owner. A missing REGISTER state is expected for this model and is not outage evidence.
The current product sheet lists Colt and third-party access choices and a public coverage footprint, while the technical guide says partner-delivered Global SIP can differ. Treat both as product documentation. They do not prove that a country, option, peer or route is present on one account.
Documented service facts
Reviewed 2026-09-29. Compare SIP trunk services →
Authentication
DocumentedColt's February 10, 2025 external technical guide defines VoIP Access as a static-mode trunk between fixed Colt SBC and customer IP-PBX addresses. That establishes an address-bound service model, not permission to accept arbitrary traffic. Preserve the implementation worksheet, assigned SBC pair, customer source addresses, access type, ACLs and any mutually authenticated TLS certificates; confirm the exact current order because partner-delivered Global SIP features may differ.
SIP registration
DocumentedThe external technical guide explicitly says REGISTER and registration mode are not supported for VoIP Access; the trunk uses fixed source and destination IP addresses. Do not wait for a registration indicator, create a registrar object or copy credentials from another Colt service. Prove reachability, authorized source identity and call completion separately.
Inbound delivery
DocumentedColt documents configurable inbound A- and B-number formats, including national, international or extension digits, over the static trunk. Record the assigned DDI range, selected number format, Request-URI, asserted identities, source SBC and final PBX match. A working trunk does not prove that every number block or PBX route is correct.
Inbound failover
DocumentedThe guide documents single- and multi-POP resilient trunks, active/standby, load-share and proportional inbound routing, timer- or OPTIONS-based recovery and optional inbound call rerouting after all trunks are unreachable. These are ordered configuration choices, not universal defaults or proof of established-call survival. Record peers, distribution mode, timers, diversion numbers and recovery criteria, then test bounded new-call failure cases with rollback.
Number and regional availability
DocumentedColt's current product page says number porting or new numbers are available in multiple countries and describes deployments across 29 countries. The technical guide warns that partner-delivered Global SIP features may differ from Colt-network service. Neither statement proves inventory, portability, regulatory eligibility or installation time for a requested number and address; obtain a dated orderability and port result.
Audio codecs and DTMF
DocumentedThe technical guide lists G.729A, G.711 A-law and G.726 as enabled by default, with additional codecs available, recommends G.711 A-law at 20 ms as fallback and documents RFC 4733 DTMF. Codec customization, transcoding and different DTMF or sample-size handling can require explicit configuration. Treat these as service inputs: retain the actual offer, answer, packetization, telephone-event payload and RTP endpoints from authorized calls.
T.38 fax
DocumentedColt documents Group 3 fax with T.38, G.711 fallback and optional fax-related configuration, while Super Group 3 is treated as a modem call using G.711 pass-through. It also distinguishes T.38 support from T.38 transcoding. Qualify the exact direction, gateway, document set, switchover, ECM and fallback instead of transferring ordinary voice results to fax.
Signaling and media encryption
DocumentedColt's product page advertises TLS and SRTP, and the technical guide documents TLS certificate authentication plus SDES-negotiated SRTP for VoIP Access. The guide's listed versions and cipher suites are implementation details that must be checked against the assigned SBC pair and current security requirements. Verify hostname, chain, mutual-authentication behavior, selected transport, SRTP suite and downgrade behavior; capability does not prove a call was protected.
Concurrent calls and burst
DocumentedVersion 2.1 documents a default 200-concurrent-call limit and 10 call attempts per second per trunk unless otherwise specified, with increases available by request and a documented 30-CAPS maximum. It also describes combined call-admission pools and 503 or cause-34 rejection when limits are exceeded. These published defaults are not an account entitlement; obtain the ordered active-call and call-start limits and test only an authorized bounded load.
Emergency calling
DocumentedThe current product page states location-based emergency routing, while the technical guide requires a valid Colt CLI so a multisite call can be tied to the originating site and warns that invalid identity can fail or misroute. Confirm country, exact number, site mapping, authorized P-Asserted-Identity, validated civic address, callback path and provider-approved test method; never place an uncoordinated live emergency test.
Pricing model
Not verifiedThe reviewed public product and technical pages do not provide a complete account quote. Obtain dated terms for access, trunks, concurrent calls, call starts, traffic, destinations, numbers, porting, transcoding, encryption, resilience, emergency service, monitoring, taxes, support, commitment, renewal and termination. A technical default or marketing statement is not a billable entitlement.
Administration and support
DocumentedColt Online documentation says SIP Trunking reporting appears under Trunk Group, performance monitoring is an add-on and raw CDRs can be downloaded through billing; support tickets use the service or order reference. Preserve the order, handover, circuit or trunk-group identifiers, peers, redacted trust inputs, CDR and call identifiers, timestamps and ticket. Portal visibility and roles are account-specific.
SMS and MMS
Not verifiedThe reviewed SIP Trunking product and technical guide do not establish SMS or MMS entitlement on trunk numbers. Voice number service does not prove messaging eligibility, sender registration, consent controls, API delivery, throughput, retention or fees. Verify a separately identified messaging product and each exact number before representing support.
Model each trunk, POP and direction before testing resilience
The technical guide documents multiple trunks, same-POP and multi-POP designs, and up to 26 secondary or resilient trunks associated with one primary. Outbound distribution is controlled by the customer equipment. Colt documents inbound active/standby sequential routing, round-robin load sharing and proportional distribution. Preserve the exact trunk group, peer order, POP, call limit and distribution mode for each direction.
Do not collapse access diversity, SBC diversity, POP diversity and PBX-node diversity into one resilient label. A second peer on the same failed path does not establish end-to-end redundancy, and successful signaling to one peer does not prove the other peers or media paths.
Read failure detection and recovery as separate state machines
Version 2.1 documents inbound advancement after an INVITE receives no response, a TCP connection timer expires or a trunk reaches its call limit. It publishes a 1.5-second UDP INVITE retry with one retry and a five-second TCP timeout. These are guide values, not measured account behavior. A failed trunk can be blacklisted, so the first delayed call and later calls may take different routes and have different post-dial delay.
The guide documents either a 180-second recovery timer or an optional OPTIONS method that probes every three seconds and restores the trunk after a 200 response. Record which mode is actually ordered, then preserve the failed request, blacklist decision, next attempt, probe traffic and recovery before declaring failover or restoration correct.
Keep trunk failover, inbound rerouting and diversion distinct
Colt documents inbound call rerouting or partial-number replacement after every trunk is unavailable because of non-reachability. That trigger is narrower than one busy endpoint, a PBX routing failure or one-way audio. Unconditional, busy and no-answer diversions have their own conditions and destinations. Preserve the called DDI, provider attempt, selected trunk, customer-edge receipt, PBX result, diversion reason and final destination.
The voice-application page says Disaster Recovery invocation and revocation require the Voice Resiliency role and an existing add-on service. Portal visibility or a successful change request is configuration evidence; it is not proof that a live inbound attempt used the intended destination.
Negotiate transport, encryption and media independently
The current product sheet advertises TLS and SRTP. The technical guide describes certificate-authenticated SIP TLS and SDES-negotiated SRTP and says MKI is unsupported; an offered MKI can contribute to one-way media. Verify peer name, certificate chain, mutual-authentication profile, negotiated transport, SDP crypto attributes, selected codec and RTP or SRTP endpoints for the exact call.
The product sheet lists G.711 variants, G.729, G.729a, G.726, iLBC, T.38, in-band fax and RFC 2833 or 4733 DTMF. A published list is not an entitlement or quality result. Preserve offer, answer, packetization, event payload, fax transition and packets in both directions.
Bind number identity, emergency handling and operations to the account
Version 2.1 documents configurable inbound number formats and separates a Colt network CLI in P-Asserted-Identity from an optional display identity in From. Colt's current CLI material says CLIP No Screening and Presentation Number depend on national regulation and cannot be guaranteed end to end. Preserve assigned number, Request-URI, To, From, P-Asserted-Identity, privacy, diversion history and PBX normalization.
For emergency calling, confirm country, site, Colt CLI, validated location, callback and a provider-approved non-live test procedure. The voice-application page also makes portal inventory, change, resiliency and reporting access role-dependent. No account, order, trunk, peer, number, call, media, failover or emergency result was tested for this review.
Sources & applicability
Primary references for the technical details above. Operational examples and planning checklists are VoIP.info editorial guidance.
- Colt SIP Trunking data sheet ↗
- Colt SIP Trunking product page ↗
- Colt SIP Trunking External Technical Guide, version 2.1 ↗
- Colt: how to access your voice applications ↗
- Colt Companion service assurance ↗
- Colt SIP Trunking support catalogue ↗
- Colt Voice Product News ↗
- Colt CLI Presentation archive ↗
Examples require adaptation to your topology. No live PBX or hardware testing is claimed.