This number is registered to you Apple ID, but is not associated with this phone. You can keep using the number for iMessage and FaceTime until it expires.

It's not the mobile number that is expiring. As long as you're paying the bill, your carrier isn't going to shut that down. What is expiring is the iMessage/Facetime registration to that number. The phone has "realized" that iMessage and Facetime on the iPhone are registered to a different phone number than the phone number the phone is currently using. In order to maintain the Facetime/iMessage registration, the phone periodically sends an SMS to a server. If it can't send an SMS from the U.S. number, the registration will be disabled. What that means is that your daughter will only be able to use iMessage and Facetime using her Apple ID until she reactivates the phone with the U.S. number.

If her Israeli cellular plan has SMS enabled, she will be able to use that. However, people who send her iMessages to her U.S. number will not get through.

I don't know that there is any information about when or how often those blind SMSs go out. If iMessage/FaceTime do become unregistered, she can put the U.S. SIM back in and sign back into them. This assumes that her U.S. plan will allow her to send SMS messages while abroad.