Question

A newly deployed Cisco IP phone registers successfully to CUCM but every outbound call to the PSTN via a SIP trunk to CUBE fails immediately after the INVITE. In the CUBE debug, the far end returns a '488 Not Acceptable Here' response. The phone is configured with a device pool that uses a region set to allow only G.729 to the SIP trunk device pool, while the ITSP only supports G.711ulaw. What is the most likely cause of the call failure?

Accepted Answer

The region configuration is negotiating a codec (G.729) that the ITSP cannot support, producing an SDP mismatch

Answer

The phone's SIP registration expired before the INVITE was sent, causing the trunk to reject the call

Answer

The CUBE dial-peer is missing a session target, so the INVITE cannot be routed to the ITSP

Answer

The phone's NTP source is unreachable, causing a certificate time skew on the secure trunk