It is possible that your customer unsubscribed to the Tixt number and are no longer able to receive text messages.
In order to subscribe back:
1. Text the word START to the Tixt number.
2. You will receive the following update indicating that you have resubscribed and will be getting the updates:
You have successfully been re-subscribed to messages from this number. Reply HELP for help. Reply STOP to unsubscribe. Msg&Data Rates May Apply.
This indicates that you've resubscribed and will be getting the updates from the Tixt number.