Great on postpaid accounts with unlimited minutes, not so much for prepaid with limited minutes.
"There is no additional monthly charge to use the Wi-Fi Calling feature on your handset. Wi-Fi Calling uses monthly plan minutes
for the following:
Calls made from the US to US numbers
Calls made from the US to international numbers (subject to international rates)
Calls made from outside the US to US numbers (not charged roaming)
Calls made from outside the US to international numbers (subject to international rates, but not charged roaming)"
--T Mobile Customer Support Doc http://support.t-mobile.com/docs/DOC-1680
Also as noted before, this process will not touch your baseband(radio)
, it will carry over from your previous ROM.