Also do wifi calls from Android phone on VZW behind NAT, with no issues. I do have a "network extender" which has GPS link and ethernet (also behind NAT) and it does give me 5 bars around the house (up to 70mbps ish of download over LTE).
Now, your NAT setup could possibly interefere? In my case at home I have FreeBSD with pf and NAT reflection disabled by default.
