Cisco VPN client is not working with 3G modems out of the box on Windows 7. The VPN will come up, but no traffic will pass throught the tunnel.
The release notes for Cisco VPN Client 5 state: The VPN Client on Windows 7 does not support WWAN devices (also called wireless data cards).
To fix the problem you will need to update your DNE component:
For 32-bit OS: ftp://files.citrix.com/dneupdate.msi
For 64-bit OS: ftp://files.citrix.com/dneupdate64.msi
After a restart VPN will work fine.