This is a cool thing which works with the newer mobile phones that come with a IMPS compliant presence-enabled phone chat client. Fortunately an installable client seems to be available for us still stuck with older phones.
Basically it will be like an integrated (for the newer phones) instant messenger client, which will let you see other users’ presence status from e.g. the address book before making a phone call. So one can see that a person is unavailable before calling and save the need to make the call.