David,
There is no "expansion" per se. However, as I recall, there were three variations of the voice messaging module, as follows:
1. 2 Port; 5 Hour
2. 4 Port; 10 Hour
3. 8 Port; 20 Hour
So, if you have one of the smaller units, you could "upgrade" by purchasing and installing a larger capacity one.
Keep in mind, however, all greetings and messages stored in your existing module will be lost, permanently, upon removal and installation of the replacement.
Good luck!
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