Where the entrance is supervised, where the risk profile is low, on a like-for-like replacement, and where cost across many dwellings is the deciding factor.

If a concierge or reception controls the entrance, the identification that video provides is already being done by a person, and the handset is simply a means of speaking to a resident.

In a small block with a known population and a low risk of unwanted entry, the security difference is smaller than in a large anonymous building.

On an existing audio system where a single handset has failed, replacing like for like is obviously correct - upgrading one dwelling to video in an audio system generally is not possible.

And where a whole-block upgrade is being costed, the difference between audio and video multiplied by the number of dwellings is substantial.

For new build and for full upgrades in anything other than low-risk settings, video is now the reasonable default.

The same principles as any indoor station - mounting height, operable controls, and hearing-aid compatibility - and they are easy to overlook on a device this simple.

Mounting height must suit a seated user, and the same regulated dimension applies whether the device is a handset or a monitor. Because a handset is small and is often fitted at a convenient height for the installer, this is frequently missed.

Controls must be operable without fine manipulation or tight grasping, which means a door release button that is large enough and requires modest force.

Hearing-aid compatibility matters directly for a device whose entire function is audio. An inductive coupler in the handset, or a compatible amplified handset, may be required by the applicable standard.

Visual indication of an incoming call, alongside the audible ring, serves residents with hearing impairment and is often required.

Check the standard for the market and specify these explicitly rather than assuming a standard product complies.

Considerably more so, because there is far less in them.

An audio handset contains a microphone, an earpiece, a ringer, a switch or two and a small amount of circuitry. There is no display to fail, no backlight to dim, no camera path, no image processing and usually no software.

The practical consequence is very long service life. It is entirely normal to find handsets working after several decades, and failures when they occur are usually the handset cord or a switch rather than the electronics.

A video monitor is a more complex device with a display that has a finite life and firmware that may need updating, and it is more sensitive to power and cabling quality.

That reliability difference is worth weighing on large blocks, where the maintenance burden is multiplied by the dwelling count - though it does not outweigh the identification benefit where security is the driver.

Simple multi-wire or two-wire bus cabling, and they are tolerant of long runs and older installations.

Traditional audio systems use a small number of conductors from the entrance panel through a riser to each dwelling, with a common bus and individual call wires, or a two-wire arrangement carrying everything on one pair.

Audio is undemanding: the bandwidth is low, so cable quality and length limits are far more forgiving than for video, and installations frequently work on cable that would be unsuitable for anything else.

That is why an existing audio riser is an asset in an upgrade - a two-wire video system may be able to reuse it, which transforms the cost of adding video.

When replacing handsets in an existing system, confirm compatibility with the controller and entrance panel rather than assuming a similar-looking device will work: systems from different eras and manufacturers use different signalling even where the cabling looks identical.

Often yes - many video systems support audio-only stations alongside monitors, which is useful for secondary positions and for cost.

A hybrid arrangement lets a dwelling have a video monitor at the main position and a simple audio handset elsewhere - a bedroom or an upper floor - so a call can be answered and the door released without a second monitor's cost.

It is also used where a particular dwelling does not need video, or where a phased upgrade fits video to some dwellings and retains audio in others.

What must be confirmed is that the system supports mixed stations, how many devices a dwelling may have, and whether the audio station can release the door or only answer.

Compatibility is manufacturer-specific, so this is a question for the system supplier rather than an assumption - and it is worth asking before designing a phased upgrade around it.

The cord and the switch hook, far more often than the electronics.

On corded handsets the curly cord is the item that fails: repeated flexing eventually breaks conductors, producing intermittent audio or none, and the fault moves as the cord is handled - which makes it easy to misdiagnose as a system problem.

The switch hook or cradle switch is next, since it operates every time the handset is used, and a worn one leaves the device permanently off-hook, which can block the bus for other dwellings on some systems.

Door release buttons wear with use, particularly in blocks with heavy traffic.

Physical damage from being dropped, and cable damage at the wall entry, account for most of the rest.

Because these are cheap parts, holding a few spare handsets is usually more economical than fault-finding at the device level - and it makes a dwelling's service call a five-minute swap.

Usually yes for security, and the cost depends almost entirely on whether the existing cabling can be reused.

The security argument is real: identification by sight rather than by the caller's claim is a meaningful difference, particularly for vulnerable residents and in buildings where entry gives access to every dwelling.

The cost argument turns on cabling. If a two-wire video system can run on the existing riser, the work is the entrance panel, the controller and the indoor stations, and the disruption inside dwellings is limited to swapping a device. If new cable is needed to every dwelling, the cost and disruption multiply enormously.

So the first step in any upgrade assessment is a cabling survey and a test on one riser.

It is also worth considering a phased approach where mixed stations are supported - upgrading vulnerable residents' dwellings first while retaining audio elsewhere.