Motivation:

Dave and George are best friends. They hang out together a lot. Sadly, one day Dave has to move to a different city. Seemingly they can still chat over the phone. However Dave is mute and George is deaf… Is their friendship doomed now?

The feature:

A SIP client (either a soft phone or real phone), might be capable of supporting mixed media streams. The user of such a device can instruct it (either in general of for a particular call) to use for example text for outgoing communication and audio for incoming one (and vice versa).

How this can be accomplished:

Figure 1 below illustrates how this can be accomplished.