Bring a Jitterbug Flip Phone to T-Mobile?

If they are not network locked, then you should be able to activate it on the T-Mobile network. There are a few considerations to keep in mind when bringing a phone to a different network. Not all phones work with all features when used on a different network. For example, if you are used to holding the 1 to call your voicemail, the preprogrammed number will not match, and you will have to dial 123 instead. The phones are also built to pick up certain network frequencies. While there is usually overlap, all the carriers also have their own frequencies, so the coverage you experience may be different than what you are used to. 

If you know someone that has T-Mobile service, you might try a quick test and put their sim card in your Jitterbug phone to see if the service works.