SIM Not Valid(Unlocked)?

So after hours of trying, I FINALLY updated my phone to iOS 7. However, it won't activate. My iPhone 4s was not factory unlocked and I had to unlock it myself in the past and am currently using a T-Mobile SIM card. I am trying to activate it with my friend's Sprint SIM card and although it worked for a few minutes and allowed me to actually use my phone, the activation required prompt kept flickering on and now it's back to activation stage. It now won't even activate with the Sprint SIM and has the SIM Not Valid error. The "iPhone was unlocked by blahblah" message still pops up and it seemed to work before. I don't know what to do now. Anyone with a similar problem possibly?