Recommendation: As posted in the link above, turning off "Block Fragmented Packets" on my home router/firewall corrected this problem right away. (You can also witness this problem by enabling and checking your router/firelogs connection log.)
Extent: This problem also seems to exist with several other WiFi access points around my area and other places I have traveled. Note: I suspect some of those WiFi-Calling problems could also be purposefully/unintentionally blocked by being too secure (such as not permitting or enabling UDP ports 4500 and maybe 500?). But many had their router in default configurations, which should have been fine with WiFi-Calling, but was not.
Discussion: It took me numerous hours with T-Mobile support, and several HTC Sensation phones to discover the WiFi Calling UDP incoming packet fragementation problem. This error magically occured after several months of very heavy WiFi calling usage without problems. Then all of a sudden WiFi Calling stopped for several days (untile I disable "Block Fragmented Packets" on my router/firewall). Now WiFi calling works, but I have created a security vulnerability in my firewall.
Problem: Without better network diagnostic equipment, I supsect that the T-Mobile WiFi-Calling server is 1) sending too large of UDP packets so must be fragemented along the way, and/or 2) not properly/adequaetly checking and/or adjusting UDP packet size based on (some offset of) packet MTU sizes (sometimes a problem when traffic is tunneled or constricted along the way with additional overhead (such as ATM, SONET, etc.)) I suspect that the T-Mobile WiFi-Calling server team could fix this on their end if they wanted (or knew) to.
Poor Man's Test: If you happen to also have an old Blackberry that can do WiFi-Calling, and you do not have accees to the router/firewall logs; then if WiFi calling works with the Blackberry and not with your (Android) smart phone, then I would expect UDP Packet Fragementation discussed above to be the problem.
Question: Please let me know if this helps or if you have other WiFi Calling solutions. WiFi Calling is very important to me since I live in a "deadish" cell zone.