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

[jrdc2p]

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?

[Ray_Harwood]

Make sure the user’s phone number is configured in your 10DLC/SMS campaign.