You probably won't find an answer, as the answers are custom
I assume you want to use Premium SMS?
The first thing that needs to be done is to find a SMS processor you would like to use.
Get their gateway specification (there is no standard, everybody uses its own implementation), then write an apropriate Payment API class.
Or make it so that each SMS gets X amount of some kind of virtual money and use this to purcahse subscriptions, etc.
If you are from Germany, I can recommend
EuraTel.