Official work phone # not recognized in Teams

Our staff has been complaining that when they use their official work phone (iPhone 7), which has their "official" work number on it, they do not get recognized as  "John Doe" when calling into a Teams meeting.    Instead, just the number shows up.   So the thinking is that "anyone" could call into the Team's meeting if they had the link to the meeting  (such as a former employee).    

Technically this is indeed true.    My question is...if have our staff's phone numbers listed in our Teams contacts, why don't their names (and avatar) show up when they call into meetings from their official work phones?     I know the "answer" will be...just use the Teams app on the phone...but that does not answer the question.  

Thanks