My last place (large IT managed services provider), was £2 per hour while on call (standby), then if you were called it was time and a half of whatever your normal hourly rate was until it was fixed. Double time if a weekend, and extra around bankhols, crimbo etc.
Was good money for me but I don't miss it all all - never really being able to turn off from work mode etc was crap.
No on call at new place - they have a system though whereby if for some reason you do have to get called (E.G the system you are an SME* for goes down), you don't have to answer the phone, but if you do then you can have £200 bonus + double time for anything you need to do to fix it. Seems reasonable - not been called out at all yet though, and probably wouldn't answer the phone if I was doing anything remotely interesting (like sleeping).
*SME = Subject Matter Expert.