Why do I have no cell Phone Service?

I have a samsung intensity from verizon and I've had it for about 22 months, so my two year agreement is about up. For the last two months ive only been able to get two bars of service, if I'm lucky. Just the other day I had zero bars every where I went and my friend who has the same phone as me had three bars. So today my mom took me to verizon and we asked an employee why i had no service, and he looked at the phone and said it's probably the battery. WRONG!! My friend and I switched batteries and he still had three bars of service and I still had zero bars!! My parents won't let me upgrade my phone/get a new one until august. What's wrong with my phone? Is there anything else I can do without paying money? PLEASE HELP!!