I'm not 100% sure, but I think OEM works like so;
Phone rings, BT picks up call, BT unit sends (pull down to GND) Mute to radio, tel-on sets radio to incoming mode (not sure what happens here, whether is a pull up, pull down or i-bus type info burst), sound then gets input to the radio unit and is then amp'd up and pushed out through the front speakers.
With the OEM setup it appears that the "tel-on" acts like some sort of handshake to prep the radio module for incoming audio.
The speed sensitive input is done via the i-bus wire.
I could of course be talking out my butt, it may work like a charm, you can try it with out the "tel-on" and see what happens.
53, M3, Silver Grey. Reto-fits: Modded Mk4 DVD, OEM Bluetooth, VR Control, Rainbow/Alpine Speakers, Sony SOT Amp, HK Sub, CCFL, Intravee II, NightMode.