Monitoring and alarm signalling

Last reviewed: 2026-03-04

What alarm signalling is

When a monitored alarm goes off, something has to carry the alert from your premises to the alarm receiving centre (ARC). That something is the signalling path: the communication link between the alarm panel and the people who respond to it.

In most installed systems, the signalling path has been the phone line. The alarm panel's communicator dials the ARC and transmits a coded message identifying which alarm activated and where. The ARC then dispatches a response, calls a keyholder, or contacts the emergency services, depending on the alarm type and the monitoring contract.

This applies to burglar alarms, fire alarm communicators, CCTV alert systems, panic alarms, and any other system that reports to a monitoring centre by dialling out.

Common signalling protocols

Alarm communicators use specific protocols to send alerts. The most common in the UK are:

• Contact ID (Ademco Contact ID). The most widely used format. The communicator dials the ARC and sends a series of DTMF tones that encode the alarm event. The ARC decodes the tones to identify the zone, event type, and account.
• SIA DC (Security Industry Association Digital Communication). Similar to Contact ID but uses a different encoding scheme. Common in commercial installations.
• Redcare. A BT-provided signalling service that uses the phone line to maintain a continuous link between the premises and the ARC. If the line is cut or fails, the ARC is alerted. Redcare has been migrating to IP-based alternatives.
• Dualcom. A dual-path communicator that uses both the phone line and a mobile (GSM/GPRS) connection. If one path fails, the other carries the signal. Some Dualcom units depend on the PSTN path and will need updating.
• CSL (CSL DualCom, CSL Live). Managed signalling services that provide IP and mobile paths. Newer versions are designed to work without a phone line.

Why digital voice affects signalling

Contact ID, SIA, and similar protocols send information as audio tones over the phone line. Digital voice compresses audio using codecs designed for human speech. These codecs can distort, delay, or drop the precise tones that alarm communicators rely on.

The problems include:

• Protocol corruption. The tones arrive at the ARC distorted or incomplete. The ARC cannot decode the message, so the alarm activation is not registered.
• Latency. Digital voice introduces a small delay in the audio path. Some alarm protocols are timing-sensitive, and if the handshake between communicator and ARC takes too long, the connection drops.
• Jitter. Variation in the timing of audio packets can cause tones to overlap or arrive out of sequence, breaking the protocol.
• Silent failure. The alarm panel may report that it sent the signal successfully (the call connected), but the ARC did not receive a valid message. Neither you nor the alarm panel knows the signal failed.

The NSI (National Security Inspectorate) has warned that alarm signalling over digital voice is not reliable and recommends that alarm companies migrate affected systems to IP or mobile communicators.

The difference between a working alarm and a monitored alarm

Your alarm system has two separate functions. The first is detection and local response: the sensors detect an intrusion or fire, and the panel activates the siren or strobe. This part typically runs on the alarm panel's own power supply and battery backup. It does not depend on the phone line.

The second function is remote signalling: telling the monitoring centre what happened. This is the part that uses the phone line. If the signalling path breaks, the alarm still goes off locally (the siren sounds, the strobe flashes) but nobody at the ARC knows about it. No one dispatches a response. No one calls your keyholder.

If your alarm is self-monitored (it calls your mobile or sends you a text), the same problem applies. The call or text may not get through if the communicator depends on the phone line.

What to do

1. Contact your alarm installer or monitoring company. Not your phone provider, your alarm company. Ask: "Our phone line is moving to digital voice. Is our alarm communicator compatible? What do we need to change?"
2. Ask what signalling path your system uses. If the answer is Contact ID, SIA, or Redcare over the phone line, the communicator will need upgrading.
3. Ask about IP or 4G communicator upgrades. Modern communicators can signal over broadband (IP) or mobile (4G). A dual-path communicator (IP plus mobile) is the most resilient option. If your broadband goes down, the mobile path still works.
4. Request a signalling test after any change. After the communicator is upgraded or the phone line is switched, the alarm company should confirm that a test signal reached the ARC correctly. Get this in writing.
5. Check your insurance. If your policy requires a working monitored alarm, a broken signalling path may mean you are not meeting the policy conditions. Ask your insurer what they need.

Fire alarm signalling

Fire alarm communicators in commercial premises, HMOs, care homes, and other buildings with fire safety obligations are subject to British Standard BS 5839. If the communicator path breaks, the building may not meet its fire safety requirements.

The Fire Industry Association (FIA) has published guidance on the impact of PSTN withdrawal on fire alarm signalling. Their position is clear: fire alarm communicators that use the phone line must be migrated to alternative signalling paths before the switch.

If you are the responsible person for a building (owner, manager, or duty holder under the Regulatory Reform (Fire Safety) Order 2005), you should:

• Identify whether the fire alarm communicator uses the phone line
• Contact your fire alarm maintenance company for an upgrade plan
• Record the change in your fire risk assessment
• Test the new signalling path and document the result
• Confirm with your insurer that the new arrangement meets their requirements

Do not treat this as a low-priority task. A fire alarm that cannot signal the monitoring centre is a fire safety failure, and it is the responsible person's obligation to fix it.

Plant monitoring and telemetry

Some commercial and industrial premises have monitoring equipment that dials out over the phone line to report faults or trigger alerts. These are less visible than alarm panels but equally affected by the switch:

• Gas monitoring and plant safety diallers, which detect gas leaks, boiler faults, or other plant failures and call a maintenance team or monitoring service.
• Water leak and flood alarm dial-out units, sensors in server rooms, basements, or plant rooms that call a number when water is detected.
• Temperature and refrigeration alarm dial-out, used in cold stores, pharmacies, and food premises that dial out when temperatures go outside safe limits.
• Building management system (BMS) dial-up modems, older BMS installations that use a phone-line modem for remote access, diagnostics, or fault reporting.

All of these use the same analogue dial-out mechanism as alarm communicators. If the device plugs into a phone wall socket, it will need an alternative connection after the switch, typically a SIM-based dialler, an IP connection to your broadband, or a modern BMS controller with built-in network connectivity.

Contact the installer or facilities management company responsible for each system. Ask them to confirm the connection type and provide an upgrade path before the analogue line is retired.

Further reading

For broader guidance on how burglar alarms, fire alarms, and CCTV systems are affected, including how to identify whether your system uses the phone line and what the risks are, see our security and fire alarms guide. If the upgraded communicator depends on your broadband router, consider a battery backup to keep it working during power cuts.