I am having problems making calls and sending text messages and now getting my iphone activated.

I have a iPhone 4 and I had problems sending texts and making calls so I got a new one. I then gave my boyfriend my old iphone snd told him if he could get it to work he could have it. For some reason we are having problems getting it to activate. it wont let us make calles to verizon or anything on it. I do still have a warranty on the phone so would I be able to send it in and get a new one? or how does that work?