Can I use my iMac's mic and speakers for a call with my android 2.2 phone (through Bluetooth)?
The reason for the Bluetooth icon being disabled is that you haven't paired the phone with a device that supports the Headset profile. Bluetooth uses profiles to categorize which services a device supports, and Macs don't support acting as a headset, only connecting to a headset (see list of supported profiles).

To use this functionality, you'd need additional software which enables the Mac to act as a headset and connect to a headset gateway (which your phone most likely supports).