All of this depends on the type of service the company offers, your contract, your country, and other stuff. For example in my company we have 5 support engine

All of this depends on the type of service the company offers, your contract, your country, and other stuff. For example in my company we have 5 support engineers who self-organized their rotation. The important thing is that someone is available from 1 pm to 2 pm to 6 pm till 8 pm, because one of our clients pays for that time range. The engineer needs to answer phone calls and emails in a timely manner, depending on the issue (from 30 minutes for issues that impact business operation to 4 hours for minor stuff, like resetting a password). They get paid a little for the time they're available and roughly the same as overtime if they actully need do something. This is in Italy and, I want to really stress this, it's how it works at my company and with our customers.