One internal user cannot SMS text a specific phone numbers, others in account can.

Individual affected has correct licensing (Person A). Was texting the phone number in question (person B) throughout the day, then suddenly messages stopped going through. 

When Person A messages other numbers, they work. When other users in our org text Person B, it works. It's only SMS messaging between (to/from) Persons A & B that are not working. This happened suddenly and with no change in the system, licensing, etc. 

Person B checked to make sure they didn't block A's number & vice versa - they haven't.

Has anyone dealt with this?