With some VoIP-provider one-way audio might occur.

Outgoing Call is possible, audio outgoing is working, no incoming audio.
Incoming Call is not possible.

Please activate this parameter in the network-settings of the PBX.
In some cases it might be necessary also to enter a STUN server here.

In the Internet router, make sure that the Port 5060 UDP (SIP) and the ports 10000-20000 UDP (RTP) are forwarded to the Gigaset Pro.