Identification, which is a security function rather than a convenience.

An audio handset lets a resident hear a caller and speak to them. Everything the resident knows about who is there comes from what the caller says, which is exactly the weakness that social engineering exploits - a claim to be a delivery driver or a contractor is difficult to test by voice.

A video monitor shows the caller. The resident can see whether the person matches their claim, whether they are alone, whether they are carrying what they say they are - and can decline without engaging.

That matters most for vulnerable residents and in blocks where entry gives access to every door inside.

Video also provides an image that can be recorded on some systems, which supports investigation after an incident.

Audio remains reasonable where entrances are staffed, in low-risk settings, and where cost across many dwellings is decisive - but video is now the default for new work.

Mounting height and reach, control size and operability, visual and audible indication, and hearing-aid compatibility - and the specifics are set by the accessibility standard for the market and building type.

The common requirements are that the device is mounted within a reachable height range for a seated user, that controls can be operated without tight grasping or fine manipulation, that indication is both visual and audible so it serves users with hearing or sight impairment, and that an inductive loop or hearing-aid-compatible handset is available.

In many jurisdictions these are legal requirements for new and refurbished residential buildings rather than good practice.

They are also among the most commonly missed items, because the device is chosen for its features and then mounted at whatever height the electrician normally uses.

Establish the applicable standard at design stage, specify the mounting height explicitly on the drawings, and check it at handover - retro-fixing height across a block is expensive and disruptive.

Largely by the cabling that exists or can be installed, which on a retrofit dominates the decision.

Traditional systems use dedicated multi-wire or two-wire bus cabling between the entrance panel and each dwelling. In new build this is straightforward. In a retrofit it may mean running cable through occupied flats, which is disruptive and expensive.

Two-wire systems exist specifically to reduce that, carrying power, audio, video and data on a single pair so an existing audio riser can sometimes be reused for video - which can transform the economics of an upgrade.

IP systems use structured cabling or the building network, which suits buildings that already have it and brings integration and remote access, but makes the door entry system dependent on the network's availability and management.

Wireless indoor monitors avoid cabling to the dwelling entirely and suit difficult retrofits, at the cost of battery or local power and radio coverage.

Survey the existing cabling before choosing.

On IP and hybrid systems yes, and it is a common requirement - but it should supplement the indoor station rather than replace it.

Call forwarding lets a resident see and speak to a caller, and release the door, when they are not at home. That is genuinely useful for deliveries and expected visitors.

The cautions are worth stating. It depends on the resident's internet connection, the building's network and a manufacturer's cloud service, so it is less reliable than the fixed device, and the service may have a subscription or an end-of-support date. Releasing a door for someone when you are not in the building is also a security decision residents should make consciously.

Data protection applies where images of callers are transmitted or stored.

So specify it where it is wanted, keep the indoor station as the primary device, and be clear with residents about what the service depends on and who provides it.

Usually several, and specifying a second is often worthwhile - particularly in larger or multi-level dwellings.

Most systems support additional monitors or handsets within a dwelling, either as full stations or as simpler slave units that ring and allow answering.

The case for a second station is practical: a resident in a bedroom or on a different floor may not reach a single hallway unit before the caller gives up, and for residents with limited mobility that gap matters.

The system's capacity per dwelling and the total across the building both need checking, since bus systems have limits on the number of devices and on cable length.

Power is a consideration too - additional monitors may need local supply or may load the bus.

Decide at design stage. Adding a second station later means cabling into a finished dwelling, which is exactly what a retrofit tries to avoid.

The door entry function stops, and what that means for egress and for access has to be designed rather than discovered.

An indoor monitor without power cannot ring, show or release. The important question is what the door does.

Egress must never depend on the system: leaving the building has to remain possible mechanically, and that is a fire safety requirement rather than a system feature. Electric locks on escape routes are arranged so they release or are overridden on power failure or alarm.

Entry is the other side. A fail-secure lock keeps the door locked without power, so residents cannot get in via the entry system - which is why a mechanical key override or a separate access control method is normally provided.

Battery backup at the system controller is common and keeps entry working through short outages.

Establish the lock's fail behaviour, the override arrangement and the backup duration explicitly, and confirm the fire strategy accepts them.

Sharing the door with a card, fob or code system so residents and staff have a means of entry that does not depend on someone being home to answer.

In most residential blocks the entrance door is used by residents entering with a credential and by visitors calling a dwelling. Those are two systems on one door, and they need to work together: the lock, the release timing, the door position monitoring and the fire interface are shared.

Integrated products handle both, and separate systems are commonly wired to a common lock with the interfaces agreed.

What needs deciding is the credential regime - who issues them, how a lost fob is voided, whether contractors and postal services get access and how that is controlled - which is a management question more than a technical one and is frequently left unresolved until after handover.

Agree it with whoever will manage the building, since an access system nobody administers degrades into permanent codes and propped doors.