Happens more often than most guys would like to think - almost every SP I know has had one (or more - lots more!) of these phone calls.
The most common one is the innocent sounding 'what's this number?' caller - a female asking what 'kind' of number this is, because she found it in her husband's cell phone on his call list, or on a piece of paper in his pocket, or, or, or. Of course, there are also the savvier women who google the phone number and know exactly what 'kind' of number it is, and who exactly it belongs to. Then you get either the irate screamer, the pitiable sobber, or the intensely curious caller.
Either way, all SPs that I know will bend over backwards to try to cover your asses, gentlemen. After all, it's in our best interest - we don't want your SO stalking us, or naming us in a divorce suit, and we honsetly don't want to get you in trouble. But some of you guys need to learn to cover your tracks better, because there's only so much we can do if your SO catches on.
Clear your call logs off of your cell phone. This is the #1 way that I know of for men to get caught. Ditch detailed billing (bills that track every incoming/outgoing call), or else buy a pay as a you go cell. Ditto voice mails - clear them, don't save them.
Use an anonymous email account, and change your password on a regular basis. The best ones can be set to automatically log you out after a certain amount of tiime.
Log off of boards when you leave - clear those cookies! It's not the dark ages - even computer illiterate women know to check your browser's history. Clear it all, every time.
Don't keep incriminating pieces of paper hanging around in your pockets, especially if you also expect your SO to wash your pants.
Have a cover story for when you slip away to see us, and for God's sake - accept that shower before leaving to go home! A suspicious woman has a nose like a bloodhound, and can detect the smell of perfume and the look of a trace of glitter from a 1/4 mile away. It's five minutes, wash yourself off.
Most of this is just common sense, but I'm sure others can add more...
Morgan