Experts enter, hours available, and their price per unit (minutes, hours, whatever).
Users would basically pay the experts premium to cut through any and all communication filters to get ahold of that person right then. If the expert doesn't answer his phone right then it would text them to respond with 3-4 digits for a set time (clients chooses time range) to reschedule (or deny).
If you find yourself not taking calls the system will automatically increase your cost per hour until it is worth your time to just answer the damn phone (it will be eventually).
Think of it as a simple handshake service for payed phone consultation.
In the speaker/entertainment industry you quickly come to realize that pretty much everyone has a price to get their full immediate attention, it doesn't matter if it's Bono or Tim Cook. Not to be crass but everyone is pay-for-play at some dollar amount.
Anyone from Ilya Grigorik, to Gary Vaynerchuk could probably set a price point that would make them very happy and generate calls.