When a call dialed to an invalid number through Service provider SIP trunk it responds with a “183 Session Progress” with Media Attribute (a): sendonly in the message body. This seems to put the line hold and play system default hold music for the user.
When analysing the call capture using wireshark there is RTP stream after the “183 Session Progress” which contains the correct recording received from the provider.
Is there is configuration from asterisk that can be used to avoid this? Or is this a issue with the service provider ?
This is expected behaviour. Sending a=sendonly is the standard way of indicating a SIP session is on hold. I’m not sure if a configuration option has been added to cope with the rare cases where the media is expected to be forwarded. I might have a look later. This certainly wasn’t an option in earlier versions of Asterisk.
Always passing media through would break holds initiated by normal SIP phones.