I had no idea that my old job had on-call support hours before I started it. Hell... I didn't even know what my job was going to be until the four weeks after I started! All I was told was that I had a generic job title of "IT Specialist", and was given an address to report to on my first day after new employee training.
After the 3 week new employee orientation class that taught me almost nothing about my actual position, I was told that they hired me to be a Lotus Notes developer. Mind you... I never even USED Lotus Notes before I started with this company! They also gave me a fairly cool two-way alphanumeric pager (cool by 2001 standards, anyway), and told me that I needed to carry it with me all the time for 24/7 support coverage. The good news was that I got to bill 4 hours of overtime for the "privledge" of carrying that pager with me all the time... the bad news was that I was a salaried employee and that I never received a dime of extra income for those extra hours until 2007. Even then, it came in the way of a settlement check from a class action lawsuit against the company, and I only got a fraction of back salary I should have earned.
The good news was that the pager never went off that often, but that's usually because I ended up having to work late in the office fixing servers most of the time. I doubt that I developed more than 1 or 2 Lotus Notes applications while I worked there, and ended up spending most of my time building and maintaining database and web application servers instead.
Anyway, that's my story about on-call support. I'll give bonus points to anyone who can tell me which company I worked for.... Odds are that you'll know if you ever worked there!