Many family court judges won't give "a hoot" about alleged adultry, but sometimes it becomes relevent to an alimony issue.
While it may be a felony by statute, not too many DA's will prosecute it. They have bigger fish to fry.
I suggest that you discuss the issue with your attorney. If he or she thinks the cell phone records would be useful, then a subpeona would be necessary. But, if they are not going to be used, then he or she may advise against it.
Your STBX does, of course, have the right to request that the subpeona be quashed. So, you have to be prepared to offer reasons for wanting these records. This one could mean extra work for your attorney (which you would pay for).
Sometimes, you have to "pick your battles" carefully.