Why does my tomtom GPS system say this?

I have a tomtom GO 150 navigation system. I can use it to call people by bluetooth on my cell phone. When I want to write a message it says this feature is not supported. What can I do to get this feature to work?? Please help!
I have unlimited texting on my phone with Cellular South

Related posts:

1. Cellular Docking Stations
2. Cell Phone Docking Station
3. Finding Someone By Mobile Number
4. TomTom One Third Edition Auto Navigation System (Factory Refurbished)
5. Unleashing The Facts About VoIP Phone Service