Could be with other brands as well, but with Fanvil, the template has the default codec list which looks like this:
SIP1 VoiceCodecMap :PCMU,PCMA,G726-32,G729,G723,iLBC,AMR,G722,AMR-WB
The problem is, that this can sometimes cause the packet length to be greater than 1500 which is a problem with some network equipment that are not configured to accept UDP packets that are greater than the standard MTU size.
In the PBX under the PJSIP settings we allow alaw,ulaw&h.264
It would be nice that the template should at least use those settings.
It would be even better if it uses that by default, and if we have set codecs on the device (under extensions) then it should use that.