Question - How do you handle PAST clients who call with a "quick question?"

We have 5-10 past clients per week calling who "just have a quick question." Our work is very detailed (insurance contracting of healthcare providers) so every client is unique and these "quick questions" aren't really such - sometimes it takes 15-20 minutes to go back and review project notes, understand the nuances, etc, and that is before the "quick question" that they actually want clarity on. Occasionally they are quick 10 minute questions, but often there is prep time, the call time and then formulating a response — so 45-90 minutes for a "quick question" is pretty common.

We've tried saying we offer a "issue specific consult agreement" for those "quick questions" but have gotten significant negative pushback, that "you did the work already, we just have questions about something that came up related to what you already did." It is eating up 10+ HOURS every week — and I dont feel that because we worked on a client 12 or 18 months ago that we should still continue to offer advice/clarity at no-further charges???

Now, some of these were great clients, and we hope to work with them again in the future, and dont want to be jerks or piss them off by not helping. (the nature of our business is such that we work on a client for a year, finish a project, and then cant really do anything again for another 24 months when we can start a new project)

Any recommendations on how you deal with past clients "quick question" issues?