How do i find out a phone number from att using a name

This sounds a bit odd.  You don't know your husband's phone number or who his service is through?

Ok, assuming this is legit and you aren't stalking some poor dude:

If he has service through ATT he will be getting a bill from them, and most phones will say ATT up on the top bar.  

Have him call you and look at the caller ID if he doesn't know his own phone number. 

No carrier will give out private information.  If he has a "burner" he is prepaying with a refill card, unless he's a complete dope and wants to get caught.  

Let me save you some time and trouble.  If you don't trust him, pack his stuff and toss him out.  Cheating is a symptom of a lousy man and bad marriage.  No state considers it grounds for divorce, not does it help a settlement.  On the other hand trust is a sign of a good marriage.

You ship is sinking sister, save yourself.