I worked with TP-LINK on some issues a few years ago. They mangled the SIP headers and did port translations that caused additional issues. We went back and forth over several months and they finally provided a beta firmware which did work after changing some settings in the router. The initial issue was that when looking at captures on the LAN, the SDP might indicate the Public IP and then a given port. Then looking at a capture of the same message on the WAN side, they changed the port to something else. As a result the media exchange was broken and oftentimes outside the firewall range. Their response was "{BTW: The randomized port translation of the TP-LINK SMB router can't be changed, we thought it can hide the real port of the internal host and can handle on different internal host that use same port". HUH? In any event not being able to manage the PAT correctly, I finally gave up as I had several different models of their business routers and they all had the same issue and TP-LINK never committed to putting out firmware for each. This all occurred in late 2014. I did not have the model you have, but the 470+. 6120, 604W and 6020. I see where they have some later firmware for some, but none of the notes mention SIP ALG or VoIP or the like, so I have not bothered with them since. I may look to see if any can utilize DD-WRT or other. In the meantime, If you can do a capture of a call on both the LAN and the WAN side, then see if the SDP gets mangled. You can then try and do the same with the SIP ALG on and see. If they do get altered, then you probably need to look elsewhere. It would have been a different story had they simply been able to handle their own port translation correctly. I have settled in on Draytek and Ubiquiti (only recently).
