Gates, staff and delivery entrances, and any door where the caller is expected and identification is not the point.

A vehicle gate is a good example: the caller is frequently a known contractor or a delivery, the distance from the reader position to the building can be considerable, and a camera at vehicle window height has its own difficulties.

Staff entrances where everyone normally uses a credential, and the intercom exists only for the occasional forgotten card, need nothing more.

Secondary and service doors in commercial buildings are the same.

Audio also suits situations where a camera would raise objections - overlooking a neighbour's property, or a position where recording would be difficult to justify.

And on retrofits, where the existing cable will carry audio over the distance involved but not video, audio is what is achievable without recabling - which is frequently the deciding factor in an occupied building.

Microphone quality and noise handling, adequate speaker output, and control of the echo path - an entrance is acoustically hostile and most complaints are about intelligibility.

Background noise is the first problem. Traffic, wind, rain and nearby conversation all reach the microphone. Noise-cancelling and directional microphones improve the ratio of voice to everything else.

Wind noise specifically is worth addressing, because a microphone in an exposed position produces roar that makes speech unusable. A recessed microphone position or a wind-resistant grille helps.

The speaker must be loud enough for the visitor to hear the reply over the same background noise, without distorting.

Echo is the subtle one: sound from the speaker reaches the microphone and returns to the person inside as an echo of their own voice. Good stations handle this electronically, and duplex handling determines whether both parties can speak at once or must take turns.

A station under a hard porch reverberates, so the mounting position affects this as much as the product does.

One per destination, with the panel chosen for the number of tenancies - and beyond a handful, a modular panel or a directory is the better arrangement.

A single-button station suits one dwelling or one reception.

Multi-button stations with two to twelve buttons cover small blocks and shared entrances, with a name label beside each button.

Beyond that, a column of buttons becomes unwieldy and hard to read, and a modular panel with a directory and a keypad is the sensible approach - the visitor looks up a name and dials a code rather than scanning a wall of buttons.

Leave spare capacity. Buildings are subdivided and tenancies change, and a panel with no spare positions means replacing it.

Name labels deserve thought: illuminated, weatherproof, legible, and replaceable without dismantling the panel. A panel where changing a name requires removing the unit from the wall will end up with handwritten labels taped over it.

Usually it means replacing the station, and often the indoor units and the wiring - so it is better decided at installation.

The station itself has no camera and cannot gain one. Replacing it is straightforward if the back box and cabling suit the replacement.

The cabling is the real question. A two-wire system carrying audio may not support video over the distance, particularly on older or thinner cable, and video on such systems has stricter distance and cable specifications.

The indoor units also need replacing, since an audio handset cannot display an image.

So an upgrade is frequently a full system replacement rather than a component change.

Where there is any prospect of wanting video, the sensible approach is to install cabling that will support it now - the cable is the expensive part to change in an occupied building - or to specify video from the outset, since the cost difference on new work is modest.

Very little, usually supplied over the same pair as the audio - which is among the format's practical advantages.

On two-wire and bus systems the station is powered from the system controller over the same conductors that carry audio and control. There is no local supply at the door and nothing to fail there.

IP stations take PoE, which is equally convenient where structured cabling exists.

The consideration is the controller's capacity and the voltage drop over the distance. A long run to a gate reduces the voltage at the station, and a station at the end of an extended run may operate marginally - which shows as poor audio or intermittent operation rather than a clean failure.

Check the manufacturer's distance and cable specification, and use the specified cable rather than whatever is available. Bell wire and alarm cable are common substitutes and are the usual cause of a system that works on the bench and not on site.

In many public and commercial buildings it is required, and it is good practice generally - a hearing aid user cannot use a door station without one.

An induction loop transmits the audio magnetically to a hearing aid switched to its telecoil setting, delivering speech directly without the background noise the aid's microphone would pick up. At a noisy entrance that difference is decisive.

Some door stations include a loop; otherwise a small loop amplifier is fitted at the panel.

The requirement is set by local accessibility regulation and varies by jurisdiction and building type. Public buildings, healthcare, education and larger commercial premises commonly require it.

Where a loop is fitted, it should be indicated with the standard symbol so users know it is there, and it should be commissioned and tested rather than merely installed.

A visual confirmation that the call has been answered is the complementary provision, and matters for people who are profoundly deaf and do not use a hearing aid.

Periodic checking of the seals, the buttons, the acoustic performance and the name labels - and it is usually neglected until somebody complains.

Check for water ingress at the cable entry and behind the panel. Corrosion inside a station is progressive and eventually causes intermittent faults that are hard to diagnose.

Buttons wear. A button that has become unreliable produces calls that sometimes fail, which residents interpret as the system being broken.

Test the audio in both directions at the door, in realistic conditions rather than a quiet moment. Grilles clog with dust and insects, which reduces both microphone sensitivity and speaker output.

Name labels fade in sunlight and become illegible, and illumination fails - both of which make the panel unusable for a visitor at night.

Where the station is on a gate, check the mechanical fixing as well. Gate-mounted equipment is subject to constant vibration and the fixings loosen.