Is This Cheating? I Think So...

A week ago I was checking through my boyfriend's cell phone and found a number I've never seen before. He has been applying for new jobs and at first I thought it was a job recruiter. I was still curious so I called.

A girl answered and said "what's up sauce? how's the ball traetin' ya?" in a cutesy voice and I hung up I now realized it was this girl from his old job that he told me about that called him sauce when he dropped a piece of pizza in his lap.

He said she wasn't cute but she sounded cute on the phone and it sounded like they've been talking. If he thought that then why didn't he tell me he called her? And why isn't her number in his phone list (he typed it in).

I think it's cheating and I'm mad about it. He says it's innocent.