Needs subscription. There is an M-Budget subscription for 10/month, cancellable at any time.

A Digitec or Sunrise subscription at 10CHF will do the trick for my daughter. But you need a SIM card with calls and data.

It simply has to be a SIM card with a phone number. At first I only had a SIM card with internet (like the iPad) but that doesn't work. The watch can make phone calls and write text messages and therefore the phone number. Whether it's a plan or prepaid probably doesn't matter.