I just bought a new T-Mobile G2.  I wanted to turn on Wifi calling but kept receiving "W006.15 isp or T-mobile Network Error" and asking me to register. 

After doing some research I looked at my cable modem/router logs and config.  There were fragmented packets originating from the phone and the incoming fragmented packets were blocked.  I disabled the "Block fragmented packets" setting and the phone was able to connect for Wifi calling