Zen Internet SIP Trunking: registration, media and continuity boundaries

Identify the selected service and order

Zen presents SIP Trunking alongside Digital Voice and CloudComms, and describes it as the option for retaining an existing compatible IP PBX. Record the legal account, ordered service, numbers, allocated channels, PBX or gateway, Zen connectivity dependency, support level and current portal values before applying a template.

The reviewed specific terms are still published by Zen but carry an issue date of March 3, 2011. Treat them as evidence of the published Business Talk SIP terms, not proof that a current customer accepted that exact version. Preserve the dated order and all incorporated terms.

Bind registration and firewall rules to evidence

Zen publishes voip.zen.co.uk, UDP 5060, Digest authentication, proxy use, no STUN and re-registration up to 900 seconds. It lists signaling from 212.23.7.228 on UDP 5060 and media from 62.3.88.0–62.3.88.31 on UDP 20000–30000. Apply least-privilege rules to the selected topology rather than opening broad internet ranges or exposing PBX management.

Retain DNS, transport, source and destination, SIP and authorization identities, Contact, expiry, challenge and response. A successful REGISTER does not prove inbound or outbound calls, and a failed call does not by itself identify authentication as the cause.

Prove routing, identity and rerouting separately

Zen says customers can keep existing numbers and that calls can automatically reroute when an office is offline, but the current product page does not publish the trigger, timing or restoration sequence. Preserve the number order or port result, portal route, inbound Request-URI and identity headers, PBX match and final destination for normal delivery.

For continuity, record the configured trigger, alternate site or device, precedence, capacity and return-to-primary behavior. Test new-call failure and recovery in an approved window. Do not infer active-call survival, caller-ID preservation or a detection interval from the availability of automatic rerouting.

Resolve media, DTMF, fax and protection on the call

Zen’s general settings list G.711 A-law and mu-law with 20 ms packetization, RFC2833 and in-band DTMF, G.711 fax, no T.38, and B2BUA media handling. Preserve offer and answer SDP, negotiated codec, telephone-event mapping, RTP endpoints, decoded audio or fax result and direction.

The same page lists UDP but not TCP, TLS or IPsec, and does not establish SRTP. Do not imply encrypted signaling or media from authentication alone. Verify that source filtering, PBX credentials, management access, fraud controls and monitoring each address their own risk.

Accept emergency and connectivity dependencies

Zen’s specific terms say 999/112 carriage is not guaranteed, advise an alternative, and identify power, broadband, suspension and termination as failure conditions. They also require updated location information and warn that emergency services may not receive it. Confirm which terms apply, the registered address, callback identity, nomadic policy, power and network dependencies and an accessible alternative for every site. Never place an uncoordinated emergency test call.

The terms also say shared data activity can impair the service and allocate internal-network configuration to the customer. Treat access capacity, contention, QoS, power and provider service health as separate evidence paths.

Reconcile capacity, price and acceptance evidence

Zen markets adjustable channels, while the terms restrict use to the allocated channel count. Obtain the selected concurrency and call-start limits, and do not infer them from scalability language. The current product page requests contact, so preserve the dated quote, charges, taxes, destination rates, minimum term, cancellation treatment and support commitment rather than repeating historical figures.

Build a bounded acceptance matrix for registration, inbound, outbound, identity, audio, DTMF, fax, rerouting, power or access failure and rollback. Record expected and observed results without turning published compatibility language into a laboratory claim.

When the Zen trunk registers but audio is missing

Use the Zen media-path diagnostic to separate registration, dialog establishment, SDP, the published B2BUA media ranges, both RTP directions, codec selection and decoded audio.