In case you wonder why your device does not connect as Modem via Bluetooth to the Mac OSX, or you cannot see COM port on Windows, your mobile device may not simply support or provide DUN (Dial Up Network) and/or SPP (Serial Port Profile) profile.

On Mac OSX I got following error, until I tried another device and/or starting by hand DUN service on a mobile/client machine:

cannot open /dev/cu.Bluetooth-Modem: Resource busy

Notes:

• Read Bluetooth specification for Serial Port Profile.
• Read Bluetooth specification for Dial-Up Network Profile.
• Modern mobile phones / USB 3G dongles, unfortunately, do not provide direct AT/Hayes access to the Radio Modem anymore.
• You can install DUN application for your smartphone, but it will be only a simple dummy wrapper imitating modem for simple applications, but they do not provide full modem functionality, beware, its a fake.
• Its only necessary for devices to be paired first in order to get SPP/DUN working. If device does not provide serial port over bluetooth when devices are paired and online, probably its just not there implemented.