Did either of you ever send a text to one of those "text blah-blah to some 5 digit number" to get a free ringtone, app, update on some event/service, send money to Haiti, etc...? Or entered your phone number in some website for a sweepstakes, free download, soda coupon, etc.?
If you did, it's possible that the terms of service for that download, info, etc, included them being allowed to send you advertisements or even sell your number.
No different than email opt-ins on websites really.
You both have iPhones too, heck, maybe apple sold your info. Maybe you should go all erikwgz on the apps store or something.