SIP.US trunks: identity, DID routing, failover precedence and capacity evidence

Freeze the trunk, DID and exclusive identity mode

Record the SIP.US trunk, DID, billing plan, selected authentication mode, public address or current REGISTER binding, active gateway, primary and secondary trunk assignment, PSTN Forward, PSTN Backup, channel count and PBX owner. SIP.US documents credential registration and IP authentication as mutually exclusive on one trunk.

A normal REGISTER status belongs to credential mode. IP authentication uses a stored static public address and shows an IP Auth label instead, so the absence of a registration row is not by itself an IP-authenticated trunk failure.

Keep registration Contact and IP-auth destination distinct

For credential mode, retain challenge, authenticated REGISTER, Contact, observed source alias, expiry and selected gateway. SIP.US says inbound traffic is sent to the Contact address and port shown in the registration status. For IP authentication, inbound calls go only to the configured public address and outbound traffic must originate from that address.

Enabling IP authentication causes the gateways to ignore REGISTER from the same configured public address and associate traffic with the IP-authenticated trunk. Do not repair that expected mode change by repeatedly rotating credentials.

Trace current gateway and DID assignment before the PBX

SIP.US says current traffic uses gw4.sip.us and gw5.sip.us, recommends DNS SRV through gw.sip.us, and says legacy gw1/gw2-only systems no longer register or receive inbound calls. Gateway selection remains separate from the DID primary and optional secondary trunk assignments.

Preserve DNS answers, selected gateway, trunk identity, provider attempt and customer-edge receipt. A second provider gateway is not a second customer trunk, and a registered secondary trunk does not prove the DID is assigned to it.

Apply Forward and Backup precedence without inventing triggers

PSTN Forward intentionally bypasses the PBX for all DID traffic. SIP.US states that PSTN Forward prevents PSTN Backup from engaging. PSTN Backup is conditional on no server response or no active registration and sends the call to a chosen telephone number. A secondary trunk is another separately assigned SIP path.

Record which control was configured, which one became eligible, the provider attempt and the observed result. The reviewed pages do not publish one universal SIP-response matrix, timer, retry count, restoration interval or established-call survival contract.

Treat channel capacity as a routing dependency

SIP.US says a forwarded call consumes an inbound and an outbound channel on channel-based plans. It also warns that PSTN Backup or failover through the same trunk can fail when that trunk is already at its channel limit. The capacity tracker counts limit events; it is not a time-series utilization record.

Preserve purchased channels, active calls, forward legs, the capacity alert and the exact failed call. Keep simultaneous-call capacity separate from call-start rate, PBX limits and another trunk's independent capacity.

Prove PBX, media, emergency and recovery outcomes separately

After edge receipt, retain the INVITE, default TO number form, endpoint identification, assigned context, first matching route, answer and SDP. Then inspect observed RTP endpoints, negotiated codec and decoded audio in both directions; SIP.US says its signaling gateways proxy SIP while RTP can arrive from varied addresses and ports.

Emergency caller identity, Enhanced DID, dispatchable address and notification workflow remain separate acceptance evidence. Change only the earliest demonstrated owner, repeat the same authorized call and verify rollback. No availability, compatibility, pricing or laboratory result is claimed.

Diagnose SIP.US DID precedence before changing the PBX

Use the SIP.US DID and PSTN Backup diagnostic to correlate Forward, Backup, identity mode, trunk assignment, channel capacity, PBX handling and media.