I'd do something different. You wanna find out who this koi fish is, right?
1) Phone the number from another number that can't be linked to you... A pre-paid sim or a land-line somewhere other than your house.
2) Tell them they've won something, based on a random draw of their cell number. A good one to use is cell phone credit... obviously use the number to deduce which service provider they're using (082, 072 - vodacom etc). Tell them that they've won R250 airtime in a random draw... then ask if they'd like to be entered into the draw for the grand prize. (make the prize something realistic and in line with crap you've seen advertised... like a new iPad, LED TV with blu-ray player or some crap like that.
ALL YOU NEED IS THEIR NAME AND EMAIL ADDRESS.
3) If they fall for it... you have their name. If they don't, at least you've succeeded in annoying them.
4) If you have a vindictive side, setup a gmail or hotmail email address and then register on EVERY mobile content site you can find using their cell phone number. The spam sms they'll receive will run into the hundreds!
5) If you really, really, really want to get even... place a "personal" ad in the classified section of various "adult companion" sites and magazines... write something up about being a single 19 year old nympho girl (36 -24 -34) who wants a "no strings" relationship with any guy (or girl!) and give out the number.