DIDWW SIP Trunking: inbound delivery, outbound calling and resilience research

Configure two call directions deliberately

DIDWW separates inbound trunks that receive purchased numbers from outbound trunks that send calls to the PSTN. For inbound delivery, a static endpoint targets the customer host or SIP URI; dynamic registration targets the Contact learned from the PBX or SBC. The latter can suit changing public addresses, but it needs a maintained registration and correct delivered DID format.

Outbound trunks have their own authentication, destination and caller-ID policy. The outbound SIP reference says REGISTER is supported for compatibility but is not required for placing outbound calls. Do not infer an inbound DID route from an outbound registration or authentication success.

Test routing redundancy at the right layer

For inbound numbers, DIDWW documents customer DNS SRV discovery and groups of trunks with priority, weight, timeout and selected rerouting codes. An empty static trunk port enables SRV lookup; a fixed address does not exercise that path. A new attempt can produce a separate CDR, so compare attempt identifiers when testing a failure.

Outbound DNS SRV can switch signaling data centers when one is unreachable, but that is a different dependency from a secondary customer PBX. Test each intended fault under a maintenance plan and measure new-call delivery and return to the primary route. Existing active calls need separate expectations.

Verify media and security on the actual leg

The outbound reference lists G.711, G.729, G.723.1 and GSM, while inbound trunk settings let the operator prioritize codecs. Negotiate a real test call in both directions before relying on codec overlap. DIDWW documents T.38 and G.711 fax, subject to route and country availability; verify with the actual fax endpoint.

TLS protects SIP signaling and SRTP can protect media on the DIDWW-to-customer leg when configured compatibly. The provider expressly says encryption does not persist end-to-end through other call legs. Inspect what the PBX actually negotiated and keep captures and credentials private.

Constrain emergency identity and spend

Emergency calling is tied to verified DIDs with an active Emergency Calling Service and allowed source numbers on the outbound trunk. Confirm location, caller identity and the approved test process for each operating site. The provider’s default permission for all available emergency DIDs may need to be narrowed by the operator.

Compare a real bill estimate: outbound destination minutes, numbers, inbound metered or channel capacity, fax, emergency features and any tier commitment. The public page describes outbound concurrency without a channel cap, which is not a verified calls-per-second allowance or a guarantee for the customer’s PBX.

Trace a DIDWW trunk-group attempt with current CDR fields

DIDWW now documents priority, equal-priority weight, ringing timeout, selected rerouting codes and one CDR per attempt. The DIDWW trunk-group diagnostic reconciles those controls with routing_attempt and is_last_cdr without treating a final call result as proof of the route taken.