Does VoIP work without internet?

No. A cloud phone system carries the audio over your internet connection, so when the connection dies the desk phones cannot place or answer anything. Your number is unaffected, because it lives in the provider's network rather than in your building. The useful question is where calls should go instead, and that has to be arranged in advance.

Everything below is about that arrangement: what happens by default, why it is worse than people expect, and the three things you can do about it in ascending order of cost.

Three separate things are usually blurred together.

Your phones stop. A handset on your desk or a browser tab on a laptop needs the connection to reach the provider. No connection, no calls in either direction. There is no partial mode.

Your number does not stop. It is a record in a carrier's network pointing at your provider. Somebody dialling it gets exactly as far as they always did. The failure is in the last step, the delivery to you.

Your voicemail does not stop, if it is your provider's voicemail rather than something running on a box in your office. Messages keep arriving and are waiting when you come back.

So an outage is not "the phone is dead". It is "calls reach the front door and nobody opens it", and that distinction is what makes a fix possible. It follows directly from where the switching sits, which is the subject of what VoIP is and how it works.

For the minute by minute version, including what the caller hears at each stage and why nothing notices immediately, see what happens when your internet drops.

A phone tells the network where it is by registering, and the registration is not permanent. RFC 3261, the specification for SIP, puts it plainly in section 10.2.2: registrations are soft state and expire unless refreshed. The phone re-announces itself on a timer, and in between, the record simply stands.

That has a consequence people find surprising. When the connection drops, nothing sends a message saying so. The phone is not able to, because the thing that broke is the path a message would take. So for the rest of that interval the provider holds a record that looks perfectly live, and calls are offered to an address that stopped answering some time ago.

The gap is usually seconds to a couple of minutes, not hours. It is still the reason the rule about what to do instead has to live at the provider rather than on the phone. Anything configured on the phone dies with the phone.

This is the setting most small businesses should have and most do not have. It says: if the desk phones do not answer within a few seconds, ring this other number instead, usually somebody's cell phone.

It is configured once, at the provider, on an ordinary day. It does not need anybody to notice the outage, sign in or press anything, which is the whole point, because during an outage nobody can sign in to anything.

From the caller's side, nothing happened. They hear one continuous ring and somebody picks up.

Two things to get right when you set it up:

• Point it at a number, not at a person's availability. A forward to somebody's cell at three in the afternoon is fine. A forward to somebody's cell at ten at night is a call that wakes them up, so pair it with your hours. The three usual shapes are covered in after-hours call handling.
• Test it by pulling the cable. Not by imagining it. Unplug the office router, call the main number from a cell phone, and see what happens. Ten minutes, once a year.

A router with a cell connection in it fails over automatically when the main line drops, and the phones register again over the cell network. This is the only option that keeps the desk phones themselves working, which matters if people are wearing headsets and running a queue rather than answering the occasional call.

What it buys is narrower than the brochure suggests, and it is worth being exact:

• It does not survive a power cut. The router needs electricity. Pair it with an uninterruptible power supply under the desk holding the router, the switch and one phone, or accept that a blackout takes the phones out anyway.
• It has to have room for your calls. A cell backup that carries email fine can still struggle with several conversations at once, because voice needs a steady trickle rather than a large total. Working out how much room your calls need is arithmetic rather than a guess, and it is done in how much bandwidth VoIP needs.
• Failover is not instant. Calls that were up when the line dropped will drop with it. New calls work.

A separate line from a different provider, ideally arriving by a different physical route, is the real answer to an outage and it costs what a second connection costs every month whether or not anything happens.

The test is simple arithmetic rather than a judgement about risk appetite: what does one hour of nobody answering the phone cost your business, and how many hours a year do you actually lose? If the answer to the first is large and the second is more than about one, the second connection pays. If your calls are bookings that will be made again tomorrow, it does not, and the money is better spent on the forward and the battery.

Four lines, on paper, where somebody can find them when the internet is not available to look anything up:

1. Where calls go if the desk does not answer, and who is expected to answer there.
2. Who can change that, and the phone number to reach them on. Not an email address.
3. The provider's support number, and the account details they will ask for.
4. The date you last tested it by pulling the cable.

Most businesses that handle an outage well are not the ones with redundant connections. They are the ones where the caller heard one ring and somebody said hello.

We do not supply internet connections, routers, cell backup hardware or batteries, and we have no office anywhere to send an engineer from. We cannot make a call arrive at a phone that has no way to reach the network. What we can do is decide, in advance, where a call goes when your desk does not answer, and set that up with you by hand.

The rest of the groundwork is on the VoIP basics topic page. If you want that failover arranged and tested, talk to us.