Then ask how often one is on call. One week out of every 4 weeks? 8 weeks? 12 weeks?

I share your concern - it's why I've never taken an on call job. But I've still interviewed for some and am willing to babysit a laptop once every 6-8 weeks if they typically get a small number of calls - provided there are other benefits to the job (learn something new, higher pay, etc).