I got a new job a couple months ago after staying at my old one for way, way too long. I was the only one who knew about tons of our IT infrastructure, and I anticipated that I was going to get tons of texts and phone calls from former coworkers. So on my way out I set up a formal contracting agreement to handle these questions. My hourly rate is roughly 3x my former salary rate and I bill in 15 minute intervals, always rounded up.
I found out recently through former coworkers that my old boss is really pissed off about this. I'm the first person who's left the company who hasn't just provided this sort of help for free indefinitely, and he feels like I'm “extorting” them by charging for occasional 20 minute phone calls over my lunch break. He thinks my rate is outrageously high – even though he was underpaying me so badly that my new salary is much closer to my contracting rate than it was to what I was making there.
Apparently he's been telling people to avoid contacting me unless it's absolutely necessary. Perfect! That'a a big part of why I set up this arrangement. I saw management encourage people to pester former employees constantly in my years working there. It was practically a standard way of debugging unfamiliar systems. “Just call so-and-so, he's been gone for more than a year, but he set this up.”
The funniest part is that I've had less than 8 total billable hours so far! So either it's not a money thing and he's just offended that I have the audacity to demand payment for my work, or he wishes he could contact me much more often but isn't willing to pay.
Don't work for free y'all. You don't owe shit to your former employers.