I'm not sure what voltages are sent via the Tel -+ wires but I do know that the radio interprets the signal from the phone module and send a signal via the can bus to instrument cluster on the 2009 and newer vw's with Bluetooth. Vw's between 2004 and 2008 can have a volk-L Bluetooth system added that can be controlled using the phone button on the steering wheel. Also Motorola made a Bluetooth kit that could be integrated into the factory radio.
Thanks for your reply, but unfortunately my car is a 2000 reg and so there is no factory option available which I can retrofit. I know how I can send the caller ID to the FIS LCD display once I have it, as luckily there's many people who have managed to write text to the FIS using an arduino.
All I need to figure out is how the TEL inputs to the radio work such that I can pass the call audio through the car speakers whilst at the same time interrupting and muting the current FM radio playback.
So far I have decided I will use the Sparkfun Purpletooth Jamboree: https://www.sparkfun.com/products/11924
With that I can pair the phone to the Arduino using Bluetooth and I will use two audio outputs:
I will use the mono line out to the TEL+/- pins which will provide audio to the radio on an incoming call, whilst at the same time one of the arduino pins will activate a switch to ground the TEL MUTE radio pin in order to interrupt any current playback.
The second audio output is used when performing Bluetooth streaming of music/audio and will use the stereo line out to the CD L/R pins of the radio in order to provide higher quality audio when listening to music from the phone. This second output will require switching to the CD mode but that's fine as CD will just become BT Audio, but the BT call interrupt will occur regardless of what input is selected on the radio (so there is no requirement to be on CD mode for BT calling).
That's what I have so far