How can I make the service work again?

I have a Qlink simcard. I wanted to see if it would work with my 3gs so I put the sim card in the phone and it said searching. It said this for about 2 minutes and went to no service. The weird thing is I took the sim card out and back in, and after searching for a little bit, It said home in the top left corner and had three little bars. I scurried to call my other phone and amazingly the call went through, and I could call the phone, and make calls from it and it was working like it would back in the day. After around 3 minutes, It went back to no service. So how do i make it stay in service?