You can make and receive calls except you have no audio.
Maybe your audio is fine until you transfer or until the call hits Voicemail.
The most common reason for this is a firewall setting that may be blocking your voice packets.
There is a Microsoft network preparation guide here: Prepare Your Network For Teams
However, you may also wish to check the CODECS your PBX/Trunk offers. In some cases, these can be filtered and arranged in a preferential order, from the PBX/Trunk Settings page in the portal.
You may also find the SIP ALG setting on your router/firewall is causing issues. You can read more about this Here