Verified Call
https://verifiedcall.com/
They do accept adult accounts. I'm not sure about the multi operator platform, but perhaps your client could contact them to find out what the possibilities are.
Here is an article about their entree into the adult market. Bree Olsen was their spokesmodel at the time...
Verified Call lets you talk one-on-one with former porn star Bree Olson | VentureBeat | Mobile | by Dean Takahashi