One specific person I call can hear me, but I can’t hear them. Call disconnects at 20 seconds

I can call almost everyone with no problems, but I can not call my grandson.  If I do, it rings.  He answers. He can hear me, but I can’t hear him. Then the call drops at exactly 20 seconds.   We can text each other without issue.   He just got a brand new phone and the problem is still happening.  

Neither of us is muted.  What do you think is going on here?

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