That idea is good. But did you know that SMS service takes ( near ) 50% of the money you receive?
Check those sites :
http://smscoin.com/en/
http://text2pay.com/blog/
https://www.google.com/#hl=en&sclient=psy-ab&q=sms+payment+system&oq=sms+payment+system&gs_l=hp.3..0l2j0i30l6j0i5i30l2.2433.3601.2.3908.7.7.0.0.0.0.150.726.4j3.7.0.les%3B..0.0...1c.1.5.hp.gbcJS4UXZ3k&psj=1&bav=on.2,or.r_gc.r_pw.r_qf.&bvm=bv.43148975,d.ZWU&fp=8a025fe5be7ab104&biw=1440&bih=812