I guess it is easy from some to miss my point. In normal operation, a gateway/access point is not in bridge mode. Hence my correct statement.

I could try to make sure that there is a SIP port forward in place and all necessary NAT devices.

Do you happen to know what port(s) is used for VOIPO SIP? I read that the ports VOIPO uses are as follows:
SIP Control and RTP: Port 5004 to 65000 UDP. But can't we narrow that down a bit?