Well - gee this is a tough one - but - so far, you don't know if he's called her since he moved back in. Just that he called her in August and September. Aliens can be so goofy, mine was still very much in contact with OW even as he was coming back to me. So don't make the ASSumption that this means he is talking to her now.
IF you think you can be calm and not attacking or pushing him into a corner, you MIGHT (remember, I don't know your H and how he'd react) approach him quietly and say "Gee, I found this bill for this cell phone and all these calls to OW. I realize this was before you moved home but I do find it upsetting that you have this phone and I didn't know about it. I would feel a lot better if you would cancel this phone." The key here, of course, is acting AS IF you expect that he hasn't contacted her since he moved back.
Other possibilities - keep quiet and check future cell phone bills - only works if you have access to the bills - but would give you a way to see if he's lying. OR - call him on that cell number sometime when he's out!
Tough call - but DON'T REACT - take some time so you can ACT rationally and with your goals in mind.
