Thank you for the phone number.
We checked at our gateway provider and all messages to this number were "expired". The "expired" status means, according to the documentation:
The message could not be delivered within a specific time window. For example, the destination number is not registered on the network.
This may mean that the cell phone was off (at sending time), had no connection, the text message storage was full, or the user may have configured the device not to accept text messages if the sender ID is unknown.