For a while at the beginning of the SIP revolution, presence was touted as one of the main benefits of SIP. The idea of presence is that you can set your status to busy or available, so people can know when it is OK to interrupt you with a phone call or whatever. The fundamental problem with this idea is that it is incumbent on you to keep your status updated, something that human beings are ill equipped to remember to do.

So we don't hear so much about presence these days. In any case, solutions have evolved that perform similar functions, but in ways better fitted to human nature. Caller ID is an example.

Caller ID fulfills half the promise of presence, by allowing you only to be interrupted by calls that you want to take. Instead of laboriously constructing and keeping current a whitelist of callers, you just look at the screen and make a decision. Then you take the interruption when you are ready, by picking up your voicemail. 

The other half of the presence promise, disclosing when you are available to be interrupted, suffers from a further disconnect with human nature. Setting your presence status to available gives the impression that you are sitting around doing nothing. While this may actually be the case, it is something that business people are disinclined to broadcast.

This aspiration of presence is valid, though; it really is intrusive to call somebody without any warning. It's arrogant in a way to think that the person you are calling is sitting around with nothing better to do at that moment than to talk to you. 

The solution to this is texting, or instant messaging. These are not as intrusive as phone calls, because there is no importunate ringing, and there is no 30-second deadline for your response. So you are chatting away by SMS or instant message, and occasionally -- but not as often as one might expect -- it becomes natural to agree to click on the call button and transition to real-time voice.