This is not due to a missing codec.
During the session setup, the peers tell each other which codecs they will use and assign numbers to them. That it got this far means that all the codecs were recognized, although not necessarily present. What has happened here is that the peer has sent a codec number for which Asterisk has not assigned a number.
At first sight that would be a bug in the peer. However, some versions of Asterisk had problems dealing with peers that propose a value other than 101 for the pseudo codec used represent RFC 2833 telephony events. The questions I was asking were trying to find out if the peer was broken, if this was a version of Asterisk that had broken handling of non-standard RFC 2833 media type numbers. (If Asterisk is the second party to send SDP on a transaction, it will follow the codec numbers used by the first party, but some versions were failing to update its internal list of numbers in this case.)
If the codec was known to Asterisk, but there wasn’t a suitable translator installed, you would have got an “unable to create translation path” error.