Can I use my PC as a Bluetooth headset for my mobile phone?

I just managed to do that on Windows 7 64bit using Broadcom's "Widcomm" Bluetooth stack with versions 6.2 and 6.4. There's also option to dial a phone number using Bluetooth.

Windows 7 does not support this out of the box. However, the Broadcomm Widcomm bluetooth stack does. Your device will not have this service enabled by default. To enable it, go to bluetooth devices and select the properties for your device. Under service discovery, select 'Hands-free Audio Gateway' and choose okay. A new driver will be installed and when you double click your Bluetooth device to control it, the window will now show an option: 'Listen to audio from your phone...on this PC's speakers'. You should also be able to connect from your phone to transfer audio.

Things to try:

1. Depending on the device, you can have the device search the PC for available services. Your device should pickup Audio Gateway or Voice Gateway.
2. Go to My Computer and click My Bluetooth Places. Assuming your phone is paired, double click it and you should see a list of available options that your phone supports. Right click on 'HS Voice Gateway on XXXXX' (XXXXX is the Bluetooth name for your phone). Click on 'Connect Audio Gateway'. Make sure you also add your computer to your phone by going into the Settings menu, then Bluetooth, down to Handsfree then select My handsfree then select New handsfree.
3. This article might give you some ideas:
   Use your computer to receive your mobile phone calls via the hands-free/headset profile using Bluetooth (WM5)