WiFi calling can be set up on any ROM. Not T-Mobile WiFi calling, but free WiFi calling nonetheless. There are plenty if tutorials floating around.
And it will never be built into official CM source.
Sent from my SGH-T999
Thanks for the info. I'm curious why it would never be shipped with the official ROM? The T-Mobile WiFi calling appears to be open source: http://code.google.com/p/the-ims-open-source-project-for-android/ . Is there a different reason it wouldn't ship with stock CM10 or are you just guessing?