Is it possible to switch modes (PC/Phone) Remotely on a Sennheiser wireless headset?

Yes. (but you need a lifter or EHS Cable)

This means if you are in computer mode listening to music, and a phone call comes in on your telephone you can answer it from the headset.

On hearing the ring tone (a lower pitched tone) in the headset simply push the Hook/Call Control button and the base will switch modes and answer the call. Once you end the call the headset will return to PC mode.

It does not work correctly without the ring detection, it will not work without the lifer or EHS Cable