I am in need of some advice. I apologize if this has already been covered. I have searched and have not been able to find an answer to my questions! I have Iphone 4s on Sprint network and am finally out of contract. I would like to use on T-mobile and it seems SAM is a good solution although I would like to test this with my phone before committing to changing carriers. I have a T-mobile sim from a friend who is no longer using it. If I use SAM with the T-mobile sim, will I be able to go back to Sprint if necessary? Will activation with the used T-mobile sim cause an issue? Thanks for help!