I swapped out a Technicolor for a new Draytek 2860n series and hit one-way audio from the on-site PBX, the old familiar we cannot hear the customer but they can hear us scenario.

The Technicolor had SIP 5060 and 5061 along with RTP UDP 49152-49952 all configured in Port Forwarding to the local IP of the PBX, this worked fine but to rule out ISP issues as the line dropped now and then we decided to install a Draytek due to their reliability.

First thing was to disable SIP_ALG so that bad boy is switched off.
I set the port forwarding – nothing but one-way audio.
I tried Open Ports – still one-way audio.
I’ve ‘accepted large incoming fragmented UDP…’ – issues continues
I’ve disabled the ‘enable strict security firewall’ - still the issues of one-way audio

In essence the audio is leaving the building but not coming back in!!!

Any ideas/suggestions other than putting the Technicolor back in?