Please check this for the end-users phone before creating a ticket:
No or bad mobile signal: This can happen especially inside buildings. The time of the next delivery attempt is chosen by the mobile network operator and can’t be influenced by us.
International roaming: Delivery can’t be guaranteed in roaming if the mobile network operator abroad does not have an agreement with the home network of the subscriber.
Portation of phone number: If a number was ported recently from another mobile network operator, it is possible that the national portation database is not yet updated and delivery will fail. In rare cases the portation databases in the destination countries are incomplete.
Blocked sender: If a sender is blocked by the user, the message will be received on the handset but not shown to the user. Delivery reports will show successful delivery in that case.
WiFi call: The handset can receive calls without a mobile signal with this feature but SMS is not supported. SMS requires a mobile signal.
Multi-SIM: If a phone number is linked to multiple SIM cards, only one device can receive SMS at the time.
Call barring/Do not disturb: If call barring is configured for the phone number by the user, SMS will be blocked too.