Unfortunately, you don't always know if on-call will be involved until you start at the job. Often many companies don't even hire you with a particular role in mind, instead you are matched to a team after you're signed on. (Especially for new grads). And sometimes on-call is introduced in an existing role, it can be difficult to refuse, especially for people in more junior roles.

But definitely ask about on-call when interviewing, in case they say they have it you can bail out before you sign.