What should I do? Is he cheating?

I have been with my bf for 3 years and I moved in with him over the summer so we can save for a house. Yesterday I found a small card that said ' Thank you for setting up your Motorla Cliq Motoblur account' and it had his handwriting on it where he jotted his name. He defensively denies that he has another phone or this account. His explanation is that it must have been an insert from when he bought a blue tooth and that he doesn't remember where it came from. However i called tmobile and they say that this phone just came out about a month ago. I feel and think that he has a secret cell phone that he talks to chick on, and he shuts off when he comes home. I dont know what to do, I do feel insecure but its for a reason!