Verizon SMS message show sent but never get there

I have had this same problem with my mom's phone for 6 years.  I would love to solve this. It is making me crazy.  Ever since she ported her number years ago, she has been unable to SMS message Verizon customers.  I have spent all day today at the At&t store with her new 14 pro.  Set it up before I got there with a new apple id, and did not restore it from the old phone, all in attempt to get this fixed.  It has not worked.  Now on hold with Verizon for the second time today.

Got At&t to add a second line. Texts go through like a dream.  What can it be?  A porting issue?