Create a virtual BT serial connection.
then use the connection setting to set up a dialup connection to that same com port as Hayes Compatible and enable terminal commands before dial out.
Note: I've not tried to do this over a BT connections but I don't see how it would be different.
If you're not using VS2005, you'll run into the fact that serial ports are not natively supported by .netcf. If you're using VS2003, and want serial port access, look for the OpenNETCF project. (http://opennetcf.org)