If the messages are not shown in the end user handset please suggest they perform the below steps in order to rule out handset related issues:

1. Power cycle the device (restart), this is to ensure a good connection to the end network,
2. Clear the inbox of the handset to determine if a full inbox is causing the messages to not be shown,
3. Remove applications connected to messaging services (I.e. disable iMessage for iPhones, message for Facebook, skype),
4. Switch the SIM into a confirmed working device,
5. Perform a factory reset on the device to ensure no third party software is causing the issue.