Can I take a phone from T-Mobile and use it as my ATT&T phone?

[beasty_narnian_vampire]

i have a phone from t-mobile but i discontinued the serice. now I have att&t, i have a phone for myself, but i want to add my family member and i dont want to buy a new phone unless i have to......so can i use my old phone that i used to have with t-mobile, with att&t??? thanx.......