[abalashov]

Not just clinics, of course. It's common practice in many businesses of all sizes.

But yes, this has been a feature of Key systems since before many HN readers were born. It was easy because the PBX knew all and controlled all; it knew the calls going through itself, and just told the corresponding phone to illuminate this lamp.

With SIP, it requires an elaborate presence (SUBSCRIBE/NOTIFY) song and dance, often implemented using competing methods, and even when you get the lamps working, cross-connecting a caller to a different phone to simulate "picking up a line" is no mean feat, since the new destination was not previously a party to the dialog. This gymnastics is usually implemented underneath using "call parking", which means users must remember to "park" calls rather than merely place them on hold, and intelligence must be devised to get line keys to map to correct "parking numbers" and so forth.

It's certainly not impossible, it just becomes an incredibly overcomplicated feat of technical jujitsu for something that worked fine for ages under the old regime.

That's because SIP was very much _not_ designed for traditional telephony. :-)