From your description, it sounds like it's going to fall under the category that Visa calls "audiotext" which is basically phone sex lines.

Since I don't think either Epoch or CCBill (or any of the off-brand billers) will offer what you are after, you'll probably have to go with a direct merchant account... but the catch is, generally, you have to have a pretty high volume, and I'm not even sure there are any US-based banks that are still accepting new customers for audiotext.

I'd say your best bet is to find a really good merchant account broker to help you. Mitch at Netbilling has some good connections, and Victoria Kozub at ECS Commerce also seems to have a really good handle on what banks are taking high-risk and adult business.