You won't find "wap billing" companies that offer just the billing part for your own products (unless you can make a few hundred thousands $ per month that could make it worth) as it is very tricky, for example the carriers need to approve every product, the landing pages, the billing flow, the content, etc and it takes months, so it's not like setting up credit card billing with a third party like epoch, ccbill, etc.
On the other hand, if you are looking into products to promote that use wap billing, hit me up, we at
www.kimia.mobi cover more than 60 countries worldwide.