The problem
Picking up a phone to find someone already talking on it is a small daily friction that no mobile ever solved — because on a mobile there is nothing to pick up.
On a house system with shared lines, it is worse: you can genuinely interrupt a call by lifting a handset.
What busy lamp field does
Each handset can watch other handsets. A lamp beside the button tells you whether that phone is idle, ringing, or in a call — before you touch anything.
Why it is quietly excellent
It turns the phone into an ambient presence display for the house. You glance at the kitchen handset and know the office is busy, without an app, an account, or anyone being tracked.
It is also the feature that most makes a house full of SIP phones feel like one system rather than several devices that happen to ring together.
Details worth knowing
Hints are generated for you. doorman render writes the dialplan hints from
handsets.toml, so the lamps cannot drift out of sync with the inventory.
It needs a phone with buttons. This is one of the few places where a desk phone genuinely beats a cordless handset — the lamps have to be somewhere you can see them.