ISP e-mails do not need to be webmaster/admin e-mail addresses.. acquiring one of those would be impossible unless you are involved in the running of said ISP. You might try reading the rules; all of the e-mail requirements are clearly laid out.
You need to use your actual e-mail address, meaning you will already need to have the login and password for it and the ability to access it. I would suggest setting it up in outlook first and making sure you can receive e-mail at that address before signing up.. otherwise signing up will be pointless if you can't get the password. :P
telenet.be is not on the blocked e-mail list, so make sure you aren't entering any spaces around it, and make sure the names in your handle start with capital letters.