I would advise against providing client with an explicit policy, and instead trying to manage it on your end without their specific knowledge.
I usually simply tell clients that while I'm happy to answer ad hoc emails and phone calls, if there is request that requires us to 'do something' such as research, development, a quick tweak, document updates, etc. then it's billable. That is a bit vague, but it works well. The rule of thumb being, if you could fill in a distinct task on a timesheet beyond 'answering emails', then bill for it.
Then rather than tasking the client with understanding your threshold for billable work, just don't engage in the constant emails and phone calls. If a client emails me repeatedly during the day, I stop answering and get back to them in single email at the end of the day. Same with phone calls, if a client constant wants to chat I'll quickly become too busy to speak with them and ask them to schedule a meeting.
But never tell them that they are 'on the clock'