I don't think you gonna find much. I think this is because there are lots of ways of payment, and they differ per country. Both in interface as the actual payment itself.
I guess most mobile game providers created their own functionality around such a payment method.
For example: my employer offers a premium billed sms service. Each sms that is sent to my employer is forwarded to the specific client. It is forwarded to the related client by HTTPS. The message format is plain XML, and contains source phone number, message etc etc. It also accepts HTTPS posts with similar kind of XML for sending SMS.
Well, do they use a *known* protocol, such as soap? NOPE! They just post plain XML.... Now, another provider may be as smart as do using soap.... And here is the first incompatibility!
So, investigation, for you, should be investigate ways to pay and their interfaces, and some kind of way to couple that to a web application for either WAP, IMODE, HTML or all if you like, depending on your target area.
But this is just what I guess ofcourse... Maybe the world is more beautiful than I think...