There are a few reasons your incoming messages are incomplete. The first reason may be the message is longer then the message length you chose when setting up the device. To change the message length you press and release either the NEXT or PREVIOUS button the device will announce numbers like 1,2,3,4. After the number you want is announced press then release the STOP/SET to store the new message length.
Another reason you messages are incomplete is the memory on the device is full. If this is the case you have to erase some or all of the messages currently saved on the device. To do this press and hold the ERASE button until you hear the device announce messages erase. release the button and all your old messages will be erased.
The last reason is you may have accidentally pressed the STOP/SET button when playing back you messages. Try playing them again and make sure you do not hit that button.