1 - it's one week length
2 - it's paid extra
3 - it's optional, but you're a bit of a bad mate if you don't participate
4 - we try to have at least 6 people on rotation to ensure a full month between on call
Because we do several changes to production per day, our coverage is around > 99% for all our services and libraries (my team is responsible for about 30 of them). We have near zero live incidents, and whenever it does happen the phone rings, it ends up being just some unpredictable spike in load that self heals without intervention.
Because on call is not painful (as it shouldn't be!) and we support each other no one has any problem being on call.