I want to use the phonenumber in an auto-login procedure, but I'm not sure how to get the users phone number.

wml-script, php, cgi?

If the user is coming in from a Phone.com gateway you can look for the "x-up-subno" user agent string that is a unique number for that particular device, but it is NOT the phone number (Sprint used to have it the same as the phone number until the Wall Street Journal complained about invasion of privacy). So the best you can do is ask them for their phone number and then maintain a database of phone numbers and subscriber IDs. Or you can put a cookie on the gateway.
If the user is running a WAP phone on a non-Phone.com gateway then I don't know, but it has been our experience that you don't get this information, and many of them don't allow cookies so you can't store that information either.