Outgoing calls are silent

We have a new Centos7 / FreePBX13 installation with Yealink W52P handsets. We’re able to receive calls perfectly but when trying to make an external / outgoing call the handset is silent (have had it ring a single time) - The call is made successfully as it rings to our mobile phones / external number and can be answered but on both ends there are no voice / audio.

We are able to phone eachothers extensions but again no audio on either handset. Only when we try to “transfer” the call do we hear the transfer message / waiting audio and if we cancel the transer the 2 handsets can hear eachother.

I am happy to provide any further info so please ask and I will be happy to provide!