International Travel, Sim Card, "This number is registered to your Apple ID, but is not associated with this phone."

I am traveling abroad for a few months and have a temporary SIM card while out of the country. Therefore, I have a different phone number at the moment then the one that is associated with my Apple ID and phone normally. I am getting a message reading "This number is registered to your Apple ID, but is not associated with this phone. You can keep using the number for iMessage and FaceTime until it expires." I have six days left on the countdown. What happens after that? I have my old SIM card that is associated with the number linked to my Apple ID and plan to pop it back in when I return home. However, for now, it is more convenient with my phone plan to use this temporary SIM. I did change in my Messages and FaceTime settings to use my Apple ID (my email) for both.

Should I just pop that old SIM card in for a couple minutes and see if that takes care of the problem? Or let my number expire or remove from the number from iMessage/FaceTime and when I return home, my number will just be associated with my sim and the phone will function as it should? Thanks in advance for the help.