Unable to call/text a family member

Really strange one I'm struggling with at the moment. I'm on a UK SIM and
my family member is on a French SIM whom I'm visiting just now. They can
call/text me, but I can't do either. Texts just disappear and calls go
straight to voicemail. They can call other Uk numbers with no problems.

I have both phones in my hands. Both are iphones fully up to date.

Where to start troubleshooting this?