My gf says shes faithful to me but I have doubts bcoz she said she was w/her sis lastnite yet her fone showed ?

that she called n spoke w/her sis for 6 mins n 30 secs. When I ask hee how is it possible she said she domt know but she is nein totally honest. she said she never left her sis side yet theres a call in the call log and she showed up at my house two hours late. I know she is insultin my integrity bcoz the fone doesnt lie unlike her. I need to leave her alone but she keep swearin n cryin that theres no one besides me, what to do