How about simply being a reasonable human being? How's that for a rule/law?
Are you really that desperate to cheap out?
Surely if you're calling someone in out of hours, then you're pulling the funds in from the customer anyway, so your company isn't really losing out?
Don't prey on peoples' desperation for pay... be at least some form of reasonable!
I used to do on-call duties for the company I work for at the moment... it wouldn't require going into the office, but could require calls and remote work at peculiar hours of the night/weekend.
We would get an extra £300/month for being on call 1 week in 6 (although it ended up 1 in 8 when a couple more wanted in on it)... then if we actually had to take a call or do anything, we would get minimum 4 hours at 1.5x rate during the week & 2x during weekends and bank holidays... I was ok with that. Didn't end up with that many calls anyway...