IP telephony: what to do during a power outage

This article was machine-translated from French and is being reviewed. Prices are in Algerian dinars excl. VAT.

When Internet access fails, what you lose with an IP telephone system depends on where the IPBX is located and how you connect to the outside. With an IPBX in your premises, internal calls continue and external calls that go through a SIP trunk stop. With an off‑site hosted IPBX, everything stops at the site. This guide describes four protections against Internet outage (a second SIP provider, a backup analog line, a degraded mobile mode, a backup 4G access), explains what each actually protects, then addresses power outage, often forgotten. IP telephony continuity is prepared before the fault.

The problem: dependence on internet

With voice over IP, calls are data packets. Their path depends on the architecture:

• calls between site stations go through the IPBX;
• calls to the outside exit via a SIP trunk (through Internet) or analog lines (via an FXO gateway).

The table summarizes what continues or stops depending on the fault.

Calls in progress at the time of a failure are lost. To choose the right protection, first identify the threat: Internet access, SIP provider, low‑voltage or internal failure (switch, IPBX).

Solution 1: dual‑SIP (backup with an alternative provider)

You open two SIP trunks with two providers, and the IPBX uses the second if the first stops responding.

What it protects: a provider or platform failure, a suspended account, maintenance on their side.

What it does not protect: an Internet access failure. Both trunks use the same access, so both fail together. To cover that too, pair dual‑SIP with a second access, e.g., 4G (solution 4).

Points to note:

• The fail‑over delay depends on the IPBX settings, especially the trunk availability test frequency. Calls in progress at failure time are lost;
• each provider assigns its own numbers. For inbound calls, a client dialing the failed provider’s number will not connect unless that provider can forward the number to the other or you publish both numbers. The backup is therefore mainly effective for outbound calls;
• You must pay for two subscriptions, and both trunks need to be configured and tested in the IPBX;

Use case: an activity that cannot operate without outbound calls and already has redundant Internet access.

Solution 2: analog backup (PSTN line)

You keep one or more analog telephone lines from the operator, connected to the IPBX via an FXO gateway. An FXO gateway links an analog line to the IPBX: each FXO port corresponds to a line.

Operation:

• The FXO gateway plugs into your local network, registers with the IPBX and appears as a trunk;
• The IPBX is configured with a backup route: if the SIP trunk is unavailable, outbound calls go through the gateway;
• Inbound calls on the analog line reach the gateway, then the IPBX, which directs them to reception or a group of desks;

Advantages :

• The analog line does not depend on your Internet access;
• If the IPBX is in your premises, the gateway and IPBX communicate over the local network: the backup works even when Internet is cut;

Limitations :

• An analog line carries only one call at a time: two lines, two simultaneous external calls. Size the number of lines for truly critical calls;
• The line has its own number, different from your SIP trunk. To let customers reach you via the usual number, the operator must forward that number to the backup line: request this before committing;
• The gateway configuration (tone detection, caller ID identification, dialing plan) requires telephony skills.

And what about a power outage? A copper analog line is powered from the operator’s central, acting as an FXS port. A simple analog phone without a power plug, connected directly to this line, can therefore keep working without power at your location. This is no longer true for the FXO gateway, IPBX and switch, which must be backed up by UPS. It is also not true if your “fixed line” is supplied by the operator’s modem or box: it stops when that unit loses power. Ask your operator what type of line you have.

Use case: a site that wants to keep some critical calls (management, key clients, emergencies) without investing in a second Internet connection.

Solution 3: degraded mode with mobile phones

If the outage lasts, the team continues working with mobile phones and a written procedure: who calls whom, which number is given to clients, what message appears on voicemail.

Advantages :

• no additional equipment;
• easy to set up.

Limitations :

• Outgoing calls display the mobile number, not the standard;
• no centralized call log, no queue, no shared voicemail;
• incoming calls to the standard number do not arrive, unless call forwarding on the operator side: if your operator offers forwarding of the main number to a mobile, ask how to enable it during an outage.

Variant: with an off‑site hosted IPBX, some providers offer a mobile app. Employees then continue to receive standard calls on their smartphone via 4G, even when the site’s access is cut. Office desks and internal calls remain unavailable.

Use case: a small team for whom a simple procedure suffices.

Solution 4: fixed 4G as backup internet

A second Internet access via a 4G router, paired with a dual‑WAN router, takes over when the primary access fails. The switch is automatic (failover) and standard traffic goes through 4G.

Advantages :

• it protects against a failure of the primary access, even for an off‑site hosted IPBX;
• it combines with a second SIP provider (solution 1).

Limits and checks:

• Independent outage: choose, if possible, an operator different from the primary access. An outage of a single operator’s infrastructure would affect both accesses.
• IP Address : in 4G, the public address often changes and may be shared with other subscribers. A SIP trunk with registration (the standard registers at the provider) usually reconnects automatically. An IP‑address authenticated trunk will not work with a new address until the provider authorizes it. Ask your provider before choosing.
• Quality : transmission delay and jitter vary with mobile coverage and load. Measure them on site with trial calls before relying on this backup. A low‑bitrate codec such as G.729 eases a narrow link at the cost of lower quality.
• Data Volume : a G.711 call consumes about 87 kb/s each way, i.e. 87.2 × 3,600 ÷ 8 ≈ 39 MB per hour each way, about 78 MB per hour for both ways. In G.729, roughly 28 MB per hour of call. Check the 4G router data plan.
• Router : it must handle dual‑WAN and automatic failover. Verify this feature on the model spec before buying.
• SIP Settings : if the router alters SIP packets (SIP ALG), calls may drop or be silent. Disabling this feature is the first test when problems occur.

Use case: a site with primary fiber or ADSL access, where an outage penalizes activity.

The electrical current: the forgotten link

The entire telephone chain depends on low‑voltage: the IPBX, the PoE switch that powers the workstations, the router, the modem or operator box, and the FXO gateway. Each of these elements must be powered by a UPS. The autonomy is not obvious; it must be calculated.

1. Add the wattage consumption of everything the UPS powers. For the PoE switch, count the actual workstation consumption, not the switch’s maximum PoE budget.
2. Read the UPS datasheet for its battery capacity: energy in Wh equals voltage multiplied by capacity in Ah.
3. The order of magnitude for autonomy in hours is energy in Wh divided by consumption in W. In practice it is shorter due to UPS losses and battery aging.

Example calculation with chosen figures for illustration, not from a model: a 100 Wh battery powering 50 W gives at most 100 ÷ 50 = 2 h.

The shop displays the onduleur EAST UPS EA265 Pro of 650 VA / 390 W at 10,500 DA HT (06/11/2026, subject to availability). The datasheet gives its power, not its autonomy for your load: ask for it or calculate it with the battery capacity. The overall sizing is explained in Onduleur : quelle puissance choisir. An UPS does not fix a fault with your operator: if the neighborhood is cut off, your Internet access may be too.

AsloTech recommendation

We recommend proceeding in this order:

1. Define what must continue absolutely: inbound client calls, outbound calls, emergency calls, internal calls.
2. Put the IPBX in the premises if internal calls must survive an Internet outage, and protect the IPBX, PoE switch, router and modem with an UPS. AsloTech supplies and installs Avaya, NEC and Panasonic IPBXs; the available backup functions depend on the model and its licenses.
3. Add external backup as needed, according to the table below.
4. Write a fault procedure: who notifies clients, which mobile number is used, where the UPS is located, and who contacts the operator.
5. Test the backup: disconnect Internet access, then simulate a power outage by unplugging the UPS from mains, and each time verify incoming and outgoing calls. AsloTech can perform this commissioning test at service activation. Repeat this test regularly (see Preventive or corrective maintenance).

The sizing and price of a backup depend on your site: we quote them in a proposal.

Frequently asked questions

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