Have you updated the Communicator Client on your mobile device?
http://www.microsoft.com/downloads/details.aspx?FamilyID=2eea3e24-f216-4887-92b0-f37d942e26e0&displaylang=en
You also have to change the ClientVersionFilter on the EDGE and Front-End Server
For some reason the RTM version of Communicator Mobile is lower than the allowed version
Step 1. Open Your Standard of Enterprise Pool
Step 2. Open your Client Version Filter (Option in your pool configuration.
Step 3. Change the CPE value to 2.0.387.* >= Allow
Step 4. Stop all started services
Step 5. Start all stopped services
Step 6. Connect to your Access Edge Server search your ClientVersionFilterConfig.exe (location: C:\Program Files\Common Files\Microsoft Office Communications Server 2007)
Step 7. Repeat step 3.
Step 8. Stop all started services (on your Access Edge Server)
Step 9. Start all stopped services ((on your Access Edge Server)
Step 10. Reconnect again and it works! Succes!