I replaced my pfSense with OpenWRT a week ago, and I just now realized that incoming calls are not working - neither for my SIP PBX nor for regular FaceTime calls.
SIP has the option ping mechanism for sending keepalives to the SIP Proxy server on then internet. These option ping messages are supposed to keep the firewall open in the inbound direction. FaceTime probably has some sort of keepalive mechanism also because nothing needs to be done in modern consumer-grade routers to allow inbound FaceTime calls. The issue with the inbound calls not working is definitely correlated to me replacing pfSense with OpenWRT.
Today my son's piano teacher couldn't call in via FaceTime like he usually does. When my son tried to call him back, the call from the piano teacher came through. So, to me it seems as though the outbound FaceTime call opened up the required ports for the inbound FaceTime call to come in. But, these ports do not stay open long enough until the next outbound kepalive is sent.
So, what can be done with OpenWRT's firewall to keep the ports opened by the outbound keepalive messages from closing up too quickly?