One Contact calls and gets dropped as soon as I answer!

I have a OnePlus 13 that drops the call everytime from only one contact only.  That person has an iphone and it doesn't happen on their end with any other person except me.  I have done everything (new sim card, factory reset etc ) with no resolution.  [Edited: "Language"]? Since Xfinity does not sell or support OnePlus phones, I have gotten zero in-store assistance - I had to get the new sim card via online support. Any suggestions are welcomed!