Presumably you are asking how to buy bitcoins using SMS where the payment goes through the mobile provider?
Carrier billing is a very expensive method to acquire bitcoins. But if the size of the purchase is small enough the convenience using that method may outweigh the higher cost.
In most areas, there are preferable methods for buying bitcoins. In the U.S., cash can be deposited at a bank and the link to claim the bitcoins would arrive via e-mail thanks to that service by BitInstant and their partner Coinapult.