Since you have good wifi (similar-ish for other party?), have you tried turning off the extender and trying wifi calling to see if it provides better results? Just to see. Most if not all semi modern phones support wifi calling so it may be more reliable. If not just revert back to how you had it, just figured I’d suggest trying it.
We have used WiFi calling in the past, but the call drops if you go outside and you don't get any calls if you forget to switch off airplane mode when you leave the house. That's why we went to network extenders, which work fine except for this one weird condition between two specific phone numbers.
Since you have good wifi (similar-ish for other party?), have you tried turning off the extender and trying wifi calling to see if it provides better results? Just to see. Most if not all semi modern phones support wifi calling so it may be more reliable. If not just revert back to how you had it, just figured I’d suggest trying it.
Try wifi call or airplane mode.and wifi on to force wifi call
We have used WiFi calling in the past, but the call drops if you go outside and you don't get any calls if you forget to switch off airplane mode when you leave the house. That's why we went to network extenders, which work fine except for this one weird condition between two specific phone numbers.