Switching sim card (iPhone X)

My son lost his phone, unfortunately, so I am going to order his new one next week. My question is: if I allow him to put a new sim card in my phone what will happen if someone tries to call me on my regular number? He will have a different number on his sim card so I'm not sure if people will be able to leave a voicemail. Will it just keep ringing or what, if anyone knows, will happen until his comes? Thanks, I'm trying to be a nice and very understanding parent.