My samsung soltice 2 gets no bars!!!???!!?

I just bought a samsung soltice from my friend. Worked fine has full bars when he showed me. I buy it put my sim in and no bars we were at school and they have cellphone jammers so I thought nothing if it. Went home same thing! I live right next to an AT&T tower and that my company I tried my sim in my old phone an it worked ful bars. Back in the solstice and nothing! Sometimes I get bars for like a minute then it cuts out again. When I try to send a text it says service unavailable. Please help! I can't take it to AT&T either because I'm under contract on my other phone.