Amy, 
This is how we handle on-call:
The schedule is rotated on a weekly basis by all the techs.   (Since we have so many, most are only on every other month).  We are paid $1.65/hr to  carry the pager while not needed at work.  In other words, if an on call tech gets off work at 4:00pm, but someone is in the lab until 9:30 pm, the call starts at 9:30pm and covers until the first tech comes in in the morning.  On the weekend you'd get paid for 24 hrs on Sat and Sun, as long as you weren't called in.  If you do get called in, your on call pay stops because you are back on the clock at work.  If you have worked 40 hrs that week, you would get time and a half for hour worked.

Beth Morehead
Med U of SC
Chas SC