I use a SIP phone from home over an internet connection to connect to a PBX in the internet. The internet connection is 50 Mbps down / 10 Mpbs up.
Empirically I have tested several routers and found some router/firewalls allow the SIP traffic to pass through the firewall successfully while others do not allow the SIP traffic to pass, regardless of the settings (SIP ALG on/off, etc).
What qualities of a router/firewall prevent the SIP traffic from transiting?
How can I look for these qualities when I shop for hardware?