SMS is old content, and most people who are currently working on the game...have no clue how SMS works, to be honest. It gets buggier and buggier with every release...and more and more, we are planning on throwing the phone into the bay for the sharks.. and rebuild it from the ground up...
but outside of that... typically they will call you. sometimes you can call them... if the SMS/Phone remembers their name(that's one of the bugs, we have no way of fixing right now) There is talk of possibility to just disable rando bf/gf events by default, with an option in the cheat menu to turn it back on with big red letters "Warning may cause bugs", at least until we get SMS working the way it should, and give them a better AI, so all the boys won't yell at you for not having an 'A-Cup"