I recently switched a line from Verizon to T-Mobile and temporarily used my Nokia Lumia Icon. All I did was insert the sim card, setup the APN to fast.t-mobile.com and then restart the phone.
Lock Status: Unlocked due to regulation of Verizon LTE phones as noted elsewhere
Phone calls: Works
Caller ID: Works
SMS: Works
MMS: Does NOT work
Internet: H+ and LTE randomly displayed, reached speed tests of a maximum of 25mbps
Note: Sometimes, the signal bars indicated that no signal was present even though the phone still functioned. This is noted elsewhere online as being an issue for others with T-Mobile and the Icon.
Sidenote: I'm selling my Icon and a grip of accessories on Swappa
Nokia Lumia Icon (Verizon) For Sale - UJI726 - $435 > Swappa