This is not as easy as you would expect. Basically you need to get the operators to send the a-number to you. The different operators will do it in different ways, which will cause problems. Some of the ways include that they will send it as a header, that you have to redirect to them and that they will give you a code instead of the msisdn. Basically you have to handle all operators differently.
That is, if they do it at all. Even if they do send it some operators will ask for payment to send it.
Oh, and one more thing. If people don't have their mobile phone set up for wap but have it set up for www nothing like that will be passed to you, no matter how you handle it.
I think that your best bet is to start the registration by receiving a SMS, then you will get the MSISDN. I won't say that this is an easy solution, but it is your best bet.
