The girl I'm dating thinks I know she has been cheating on me but she is not sure.?

I told her two weeks ago that when she hung up her cell it didn't. I heard her speaking to some guy. she accidentally switched it over to 3way I listen enough to know what was going on. now the past week I've called and her phone will ring 5 or 7 times before going into voice mail or she will just send it on the second ring. our conversations are bland its like she is all talked out and does not call at lunch time like she use to. Now she is asking me if I have Bugged her phone to listen on her calls i said no and she joking said I'm i a starker LOL. We have a friendship with benefits relationship and I'm cool with that so why would she be so worried if I found out? Should I just teller I know or should I let her hang her self the bad part its affecting our life. I was in it for the now thats all messed up what should I do?