Mitel PBXs have a number of default values that need to be changed in order to establish a proper bi-directional SIP trunk with Thinktels switch.
- Enable SDP on SIP Invite (off by default).
- Set DTMF to RFC2833 (out of band).
- Ensure that g729,g726 and g711 are all allowed codecs. We have noticed that Mitel PBXs are only sending g279 as an available codec out of the box.
- Ensure SRV lookups are enabled and the DNS server that the Mitel PBX uses supports SRV lookups as well.
For instructions on how to get your Mitel PBX to connect with ThinkTel please review the attached setup guide.