Thanks a lot for your help EtherealWolf.
I'm buying a refurbished Note 8 that comes without a SIM to replace my parents faltering Note 3 as a present. They are in their late 70s, not at all tech savvy and only use it for occasional calls, text and WhatsApp messaging, and the odd photo so just have a pay as you go but they do need a large screen.
I wanted to check first if they can use their existing SIM in the new phone or, if not, to get everything sorted in advance.
I will contact their provider to get a micro card