No other forum seems to cover this, so thought you folks might have a clue :>)

I have a T-mobile phone which doesn't pick up at home & they walked me thru using the AT&T network which is one of their collaborators. It still doesn't pick up at home, but much closer to it than before! However, just got my husband a Nokia el cheapo w/ another service that uses AT&T, & it picks up at home! Must be more sensitive than the Motorola I have. I like my phone, and my question is, If I wanted to try using my phone w/ a sim from the new company (after signing up, of course)would my phone work, or is it locked up so as to work only w/ T-mobile, no matter what? Am I making myself clear or is it a mess??