I need a little more information to better assist you. Is the call dropping and all connections appear to be lost, or do you still see signal? Do you get any error messages? In addition to answering those questions, I recommend you check out The specified item was not found.. Change the preference to Wi-fi preferred and test it out.
I was never able to get WiFi calling to work acceptably on the G2. I've successfully used UMA calling with multiple handsets since 2007. For 9 months I tried everything to make the G2 work, including several handset exchanges. Nothing helped. T-Mobile support was not only useless on this subject, I believe they knew the problem but simply stalled. In September I moved to MyTouch 4G Slide and Sensation and had an immediate improvement. When my G2 couldn't connect to WiFi calling, with support on the phone, after they couldn't get me connected, I could put my G2 SIM into an old Blackberry Curve and it would work every time. Put this SIM back into the G2 and no WiFi calling. This never "inspired" support to do anything further. At best, WiFi calling is not as good as UMA.
There is a lot of WiFi calling info in the below thread and in several other threads ...