Hire a reputable private investigator. Under federal law, it is illegal to purchase cell phone records off the Internet or “hack” into your spouse’s phone, but if he has not password-protected his account, it may be possible for a professional to get a record of his calls. The key is “reputable.” If you retain the services of someone who secures the phone records through questionable means, you could both be held legally liable. If you question an investigator, and he doesn’t tell you this, he’s probably not reputable.
Issue a notice to your spouse to produce documents after you have filed for divorce and your divorce complaint has a docket or case number registered with the court. Forms for this document are available on the Internet. In the notice, request the telephone records. Be specific and state the exact months you’re looking for. By law, if you have an active divorce case, your spouse is legally obligated to give the records to you as long as they are in his possession, or he’s able to get them from the telephone company.
Send a "subpoena duces tecum" to the telephone company. This form is also available on the Internet. You must have an active divorce case number with the court to use this option as well, but it’s usually more productive than trying to get the records from your spouse directly, especially if he has something to hide. Any party to the case can issue this type of subpoena; it does not have to come from a judge, which is often the case with criminal court matters. You can serve it on any third party that holds records pertinent to your case and that party is legally obligated to supply you with the records you’re asking for. You must serve a copy of the subpoena on your spouse as well, so he knows what you’re doing.
If your name is on the telephone account, you can simply call the company and request the records yourself. You're legally entitled to them. If you get the records through a subpoena, and you use the records in court, most states require that you supply the court with a copy of the subpoena and proof that you served it on your spouse, such as with a certified mail receipt.
If your name is not on the telephone account, don’t take matters into your own hands, especially if you haven’t actually filed for divorce yet. The laws surrounding access to electronic communications are complex. If you’re trying to get cell phone records rather than landline records, this is especially true, so it could be easy to step on the wrong side of the law. Consult with an attorney before you do anything to make sure you’ve got the legal right to take the action you're contemplating.