In Finland the collective-agreement that covers IT work defines this. The way I've been paid has been the same across several companies:

You get paid 50% of your normal hourly salary for every hour you're "available".

You get paid 200% of your normal hourly salary for every hour you're working on an issue, incident, outage, or problem.

So being on-call is pretty lucrative, given that you'd usually do it for a seven day period. (i.e. 9am Monday to 9am the following Monday.)