Paging Dr. Diablo, Come in Dr. Diablo

[Crocodile]

You'd need to meet him in person first. Same as you'd need to do for anyone you wanted to contact with a Communicator or such. How you get there is either gonna require a Banishment or a Void Quest may be possible (though there won't be a return gate, so you'll need to go through the big guy to leave again). After all that... well, read some of Trixie's stuff. She regularly communes with the Big D.