Jerry,
I think you should PM badbane, as well.
Need I say, be very careful to find and gather evidences.
I am lurking.
LOL just thought I would chime in. I see some red flag here with her behavior. Thought becareful jumping to conclusions. You wanna treat this like a police investigation. So chalk the behavior as a person of interest. I Think since you have a phone number you can now go to the online phone records. AT&T phone records go back 16 months. I am sure most carriers have similar logging. Look up the online logs, then sort them by phone number. See if there is a pattern of calls.
IF you think she is using the phone for her extra marital stuff. The phone records should always be the first stop. A lot of people do the dumb thing of posting their phone number online. Before you pay money on a reverse lookup.
(guys if you post a phone number online only post your business number not a personal number. If you personal number is on a business card that's okay but don't put it online or someone might post it somewhere and next thing you know you have ten thousand text messages, a full voicemail, and 250 missed calls. and a phone bill to match.)
So you go online get the phone records then you sort by number. You are looking for people your wife calls close to, or more than you. Next step is to sort the calls by duration. You are looking for any numbers that are called consistently for excessive durations. I'd say the ten minute mark is a good number to go by. Then once you have the numbers by frequency and duration, you want to determine who the number most frequently called AND the longest duration. Start from the top work you way down.
If google doesn't turn up anything just do a fake call back. I just got called from this number I was just wondering who this is and what regards the call was for. See if you can get them to give you their first and last name.
Call spoofing is kind of complex your best bet is to call from your phone since he won't have your number. Remember *69 though folks.