In the past 2 years, we have had 4 instances where we have been without power for a period of 4-6 hours - due to construction crews cutting power lines, car accidents involving downed power lines, etc.

I am trying to develop a strategy/disaster recovery scenario of what to do when power is out for more than 20 minutes. (i.e. UPS units are drained). This site is a local business with more than 250 employees. What are some things I should consider for VoIP phones?

Should I add analog lines to the asterisk box or is that a waste since the asterisk box is assumed to be not working? (Power is off)

I have absolute control over the hardware (Asterisk box) and network infrastructure. This is an on-site VoIP gateway connected directly to a PSTN T1 line.

  • Do you have any specific concerns or questions? You need to get a bit more detailed to get a good answer. – Bigbio2002 Apr 1 '13 at 19:30
  • Updated question. Basically a disaster scenario. How should somebody dial 911, etc. – AWippler Apr 1 '13 at 19:37
  • You're going to need to give more details as to what kind of environment this is (home, non-phone business, call center, etc.) and what you're trying to accomplish. – Jason Litka Apr 1 '13 at 19:38
  • 3
    Get a bigger UPS if the phones are that important. – Michael Hampton Apr 1 '13 at 19:43
  • 2
    "Updated question. Basically a disaster scenario. How should somebody dial 911, etc" - dial on the cellphone? – hookenz Apr 1 '13 at 20:31

If the UPS units are drained, there's probably not much you can do without power. Your goal should be to make sure you have sufficient UPS/generator capacity for your PoE switches and your Asterisk device to be up for however long you need in a disaster scenario (20 minutes is fairly short, I'd be aiming for an hour).

It wouldn't hurt to have some standalone POTS lines for a few emergency phones around the office.


An on-site generator is the standard answer to the problem of extended power outages. Call a local Generator company and get them to give you an estimate.

Some things you will need to figure out:

  • What are your power needs? How much power does your entire facility need? Does the entire building need backup power? Or just the server room?

  • How are the VOIP phones powered? Is it Power Over Ethernet? If so, you can get away with just powering the PoE switches/injectors. If your phones are powered from a wall outlet, you'll need backup power to everyone's desks.

  • Remember to factor in your server room cooling. Some building facilities people consider the HVAC "optional" and do not put it on backup power. This can be a nasty surprise during an extended power outage when the servers overheat and shut down anyway! In my facility, the server room is on its own cooling system that is generator-backed, but the general building HVAC is not backed up.

  • Make sure you size the generator to accommodate growth. Better to have too much capacity than too little.

  • It would probably be wise for you to review the electrical wiring in the building and figure out which outlets are connected to which breakers in which panels. Trace everything back to the main breaker panel. This may dictate where the Transfer Switch will be installed, and what will be backed up.

  • Review the brand/model of your UPSes. Any server/network hardware that is backed up by a generator should also have a double-conversion UPS.

  • Generator Run-time: Generators have a fuel tank that needs to be refilled, unless it uses natural gas (which can run indefinitely unless there is an earthquake). You will need to figure out how long the fuel lasts and schedule delivery accordingly.

  • Self-Tests: You should run regular tests of the generator, and the transfer switch. Most systems can be programmed for automated testing. Note that a full test including the transfer switch will blip the power while it cuts over, so you may not want to do this in the middle of the day. Also make sure that you have some way of knowing the self test happened and passed/failed! Some systems will plug into the network and send you e-mail alerts and reports.

  • Worst Case: Sometimes the generator will fail you. In that case, adding extra battery capacity to your UPS can be important. In our building, our phone system is protected by its own APC UPS with extra battery packs, providing ~4 hours of runtime. All of the other server/network hardware only has 20-30 minutes.


If it is for emergency cases why not use (satelite) mobile phones that are always charged or can be charged through batteries or something along that way.


You have one necessity and one option:

  • Necessity: put important equipment on generator power.
  • Option: Put important equipment in a colocation facility.

The reason that the generator would be necessary even if you choose a colocation space is that if the power to the building is knocked out, then phones won't work anyway so having your Asterix box and other important equipment humming along safely in a datacenter will do no good if your users can't get to them because your demarc room is powerless and you can't get a network connection.

You might want to consider dedicated circuits between you and the colocation facility since VOIP is involved.


Your question is a little light on detail.

So I'm going to suggest you take a step back for a moment and consider what might seem to be too obvious.

Q. When the power is out, are all or only some employees able to work?

A. Honestly, I think the answer is quite obvious. Most businesses don't have large UPS's for their computers and machinery that allow their staff to keep working.

Yet, we're so reliant on electricity there is not much you can do without it.

Since most businesses can't do much without power to computers and machinery I would think that a phone is of little use except in an "emergency" situation. Even a call centre can't do much without their computers running. You need to weigh this up against the cost of putting in a bigger UPS and providing UPS power to the phones via PoE.

Nowadays, most employees will have a cell phone which they can use in case of emergency (911 call) if the office phone system is not working.

Unless you're hosting web sites or other things I suggest you're wasting your time and money even contemplating making the phones still work in a power outage. The employees won't be able to do much anyway.

However, a UPS on servers can be very beneficial. Not having to reboot the server is a good thing if it can survive the black out period. And if it can't survive the blackout period, having the UPS signal the server that it's going to loose power soon so that the server can do an orderly shutdown is also a very good thing.

So I'd suggest you ensure your servers can keep running for around 40+ minutes off battery. I'd not bother with individual phones but if you want you could power those from PoE.

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