Start with the ordered static trunk and access path
Colt's external guide describes VoIP Access as a static-mode SIP trunk between fixed IP addresses on the Colt SBC and customer IP-PBX. REGISTER and registration mode are explicitly unsupported. Build the PBX or SBC object from the implementation worksheet: record the exact service, access type, customer source addresses, assigned SBC peers, ports, transport, DNS behavior, ACLs and change owner. A missing registration indicator is expected on this service; it is not proof of an outage.
The guide separately describes partner-delivered Global SIP, whose features may differ from trunks on Colt's own voice platform. Keep the ordered product, country, access network and handover document attached to the template. Do not use a partner or other Colt service's registrar, credentials, resilience design or number behavior to fill gaps in a VoIP Access order.
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.
Trace inbound number format and outbound identity separately
The guide documents configurable inbound A- and B-number formats, including national, international and extension-digit presentation. Preserve the assigned DDI range, configured number format, source SBC, Request-URI, To, P-Asserted-Identity, Diversion or History-Info where present, PBX match and final destination for one authorized call. A reachable peer does not prove a number is assigned or that the dial plan normalized it correctly.
For outbound calls, Colt screens calling identity against assigned number ranges and can use P-Asserted-Identity for a Colt CLI while From carries a display identity under specific features. Record the actual header contract and country rules. Do not broadly trust caller-controlled identity or assume that a display number will be presented by downstream networks.
Treat resilience as an ordered multi-trunk design
Colt documents same-POP and dual-POP topologies, up to 26 secondary or resilient trunks for one primary, and active/standby, load-share or proportional inbound distribution. Its failure handling can depend on INVITE or TCP timers, with OPTIONS probing available as a bespoke configuration, and inbound call rerouting can forward to a preselected number after all trunks are unreachable. Record the exact peers, POPs, route order, distribution mode, call limits, failure signal, timers, diversion target and recovery rule.
These controls apply to new-call routing and do not promise that an established call survives every failure. Test bounded peer, path and PBX failure cases with approved destinations, timestamps and rollback. Include the first call after failure, subsequent calls, recovery and any unexpected diversion cost or emergency-location consequence.
Negotiate media and fax from the account profile
Version 2.1 lists G.729A, G.711 A-law and G.726 as enabled by default, additional codecs that may be enabled, RFC 4733 DTMF and a recommendation to retain G.711 A-law at 20 ms as fallback. Transcoding, codec customization, different DTMF relays and sample sizes can require specific configuration. Capture the actual offer, answer, selected codec, packetization, telephone-event payload and RTP endpoints; a published list is not a quality result or proof that an option was ordered.
The guide documents Group 3 fax using T.38, G.711 fallback and separately configurable fax behavior, while Super Group 3 is handled as modem pass-through. It also distinguishes fax support from T.38 transcoding. Qualify each direction with the exact number, gateway, document set, re-INVITE, ECM and fallback rather than treating a successful voice call as fax acceptance.
Pin protection and capacity to the assigned trunk
Colt's current product page advertises TLS and SRTP. The technical guide describes certificate-authenticated SIP-TLS and SDES-negotiated SRTP, with SBC-specific protocol and cipher details. Obtain the current profile for the assigned peer, then verify hostname, certificate name and chain, mutual-authentication behavior, selected transport, SRTP suite, media negotiation and downgrade handling. Do not claim protected calling from a configured checkbox or open TLS port.
The guide documents a default 200 concurrent calls and 10 call attempts per second unless otherwise specified, a documented 30-CAPS maximum, combined admission pools and limit-related rejection. These are published technical defaults, not the customer's purchased entitlement. Preserve the ordered active-call and call-start limits separately and run only a provider-authorized bounded load test.
Bind emergency identity, operations and commercial limits
Colt says emergency routing is location based, and its technical guide requires a valid Colt CLI for multisite origin identification, warning that invalid identity can fail or misroute. Confirm the exact country, site, assigned number, P-Asserted-Identity, validated civic address, callback route and provider-approved non-live test procedure. Keep an independently tested fallback and never place an uncoordinated live emergency call.
Colt Online material describes Trunk Group reporting, add-on performance monitoring, CDR downloads and support tickets linked to a service or order reference. Retain the order, handover, trunk-group and peer identifiers, redacted trust inputs, CDR and call identifiers, timestamps and ticket. The reviewed public pages do not establish messaging entitlement, exact number availability or a complete quote. No account, order, PBX, trunk, peer, certificate, number, route, call, media, fax, failover, capacity, emergency call, message, price, availability or support response was tested for this guide.
Sources & applicability
Primary references for the technical details above. Operational examples and planning checklists are VoIP.info editorial guidance.
- Colt SIP Trunking ↗
- Colt SIP Trunking external technical guide, version 2.1 ↗
- Colt: access voice applications ↗
- Colt Online service assurance ↗
Examples require adaptation to your topology. No live PBX or hardware testing is claimed.