I don't know anything about the business structure you have in mind, but usually customers don't get to talk to the developers so they don't expect to be able to do it unless its part of the contract. In my experience usually they don't even ask unless someone has put them up to it.

E.g., they may be working on a complex issue with a Support guy in over his head who has been reduced to relaying messages to "the developers". I'm happy to jump on such a call directly because it feels like it saves everyone time and running around.

But all companies with lots of users eventually develop layers of Support in place to shield their developers from potentially constant interrupts.