Often (not always) I try to call my mother using FaceTime it calls totally the wrong person. The wrong person it calls, is always the same wrong person. How is this even possible?
Open your Contacts and check all of the phone numbers and email addresses listed for the person that gets called (check your mother's contact as well). This person's contact likely incorrectly contains your mother's phone number or email address. Remove or correct the incorrect information from that person's contact information and the problem will go away.
This has been happening in our family as well, but not sure if it's a bug in "the system" or an effect of misconfiguration somewhere (like contacts, old numbers, etc) locally.
When my daughter uses my wife's phone to call my mother-in-law via FaceTime, often a stranger will answer (seems to always be the same person) and says to stop calling. But on the phone itself is shows my wife's mom's name listed, and when we try the call again it usually goes through as expected the second time.
It happened again this morning so I tried looking it up to see if others are experiencing this and found a bunch of threads where other's have experienced this bug as well. Some other sources: