Use Kannel(Linux) or NowSMS(BillOS). You'll find it a very easy task. If
you don't wanna buy GSM modem, I suggest Billionton's USB modem (about
200K tomans) and if you wanna buy an AT enabled phone, I suggest Siemens
SL 45. You can buy a second handed one for 40-50K tomans.

If you have high traffic, I suggest you connect with one of 3 companies in
Iran whom have access to TCI's SMPP link like MAGFA or ADT.