I can think of two potential ways to get your number:
– the sim card has numbers stored on it.
– your number is listed as an emergency contact on the lock screen
I guess they could have potentially guessed the pin too or picked pocketed it unlocked…
It could easily be switched on and the message not displayed if it is somewhere that doesn’t have signal or by removing the SIM card and stopping it going online. Which will stop the Apple Find my Phone features working.
I didn’t think ‘rooting’ an iPhone was a thing these days, so expect for stripping for parts I can’t see how a stolen iPhone makes any money. Probably why they try to extort money from you for the return of the phone.
Change all password even those ‘fingerprint’ protected, just in case. Could be a good time to switch on 2 factor authentication on both your and your daughter’s accounts. They may try to target you.
I’d also check Facebook privacy settings and prevent searching via email address and mobile phone number.
Make sure it’s reported to the police and they are informed of the extortion attempts! They might one day find the group responsible for this extortion.