When using the built in SIM card slot for Text messaging, it is best to have Tobii Communicator set to "the program keeps the storage on the telephone empty". This will avoid any issues with receiving and sending text messages when the SIM card is full.
How to make that change:
1) In Tobii Communicator select Settings (in the menu bar).
2) Select Mobile Phone Settings
3) Select the Storage Tab
4) Select The program keeps the storage on the telephone empty
5) Select OK