You're not required to take the call, but you get a fairly large sum of money if you do, meaning people usually take the call despite not being required to.
Because of laws here, you're required to have 11 hours of uninterrupted "free time" after leaving work (no laws on how long a work day is), so taking a call usually means you violate that, which in turn restarts the clock on your "free time", meaning you're entitled to "sleep in" on company time the next day. Doing so of course wreaks havoc on all planning, so very few people actually take the full 11 hours off, and instead just get in "late" around 9am.
Edit: I should probably have explained that the calls happen on average once per system every 2-4 weeks, and the amount of people on call is probably limited to <5%-10% of the engineers.