You've created your own problem by A) being a nice person (no good deed goes unpunished) and B) being immediately accessible by phone. If you were at a party where a medical doctor was present and if you went up to the MD and said "Hey, what is your diagnosis for my symptoms?' the Doc would say "Make an appt." Same goes for your practice meaning you get to choose when you call them back. It might be cost effective for you to get an answering service or just always use an answering machine to avoid getting "buttonholed" when you answer the phone yourself.
Once you call the person back, you can control the introduction by saying something like " Hi, you called and had a question. If its a quickie that you can describe in two minutes that I can answer right now then this may be a freebie, but a complex one is billable. OK?" Then everyone is on the same page.
No doubt the folks who are used to always getting freebies will be unhappy. Some may leave. But there is no point in working for free.