Locking a Sprint phone to T-mobile

[nootrak4]

I did the SIM unlock. I got a Tmo sim and i would like to know how or is there a way to lock my sim to tmo or other carrier. I would love to do this due to the fact that every time i switch over no matter what i have to keep on registering to the tmo network. I put my sim in a tmo phone and it works with no problem. no disconnects or anything. does the IMEI Number need to be in Tmo's computer. does this work just like sprint. anything will be greatfull.