The SIP Trunk Plus platform supports the following CODECs:

G.711a (PCMA)

G.711u (PCMU)


Please note that the following CODEC’s are not supported:


You should therefore ensure that when setting up your SIP trunk, in order to avoid any 'transcoding' issues (refer to 'I Can Make Calls To Some Numbers/Destinations But Not Others'), that you configure your SIP trunk to reflect the above CODEC's.  For example, in 3CX, set the 'Assigned Codecs' at shown below: