I'm happy to report that it worked. My sister visited with mum in Scotland, got a UK SIM and put it in an old iPhone, entered the few contacts we needed, then set up Alexa and the Echo Spot, logging in to mum's Amazon UK account.
I entered that number as a number for mum in my phone contacts. After restarting my Alexa app, it recognized her as an Alexa user, and we were able to enable "drop in" with each other.
Call quality is great, and it's loud enough for mum to hear me even without her hearing aid. We sat the Spot right next to her chair, on her coffee table. She loves it, and has already "dropped in" on me several times today.
This is a great feature, for elderly relatives at a distance. I just say "Alexa drop in on Mum" and there she is.
Hugh