What Is SIP Registration, and Why Is an Extension "Not Registered"?

Definition: what "registered" means

In a cloud PBX, the phone and the PBX are not in the same place. The PBX knows there is an extension 104, but it does not know in advance where its device is: in the office in Bnei Brak, at the employee's home, or on a mobile phone that is on a train right now.

Registration solves this. When the phone turns on, it sends the PBX a REGISTER message using the SIP protocol. In short, the message says: "I am extension 104, this is my password, and I can be reached at this address." The PBX checks the password, records the address, and answers "200 OK — registered."

The simple comparison: checking in to a hotel. You show ID, get a room, and the front desk knows where to send your calls. If you leave without telling anyone, after a while the front desk assumes you are no longer there.

Step by step

1. The phone turns on and gets an address on the office network.
2. It contacts the server address that was set in it and sends a REGISTER.
3. The PBX answers "401 — identify yourself" and sends a one-time challenge.
4. The phone calculates a response from the password and the challenge, and sends the REGISTER again. The password itself is never sent over the network.
5. The PBX checks the calculation. If it is correct — "200 OK," and the extension is marked registered.
6. The response also sets how long the registration is valid.

Note the third step: a "401" response at the start of a registration is normal and fine. It is not an error, but part of the procedure. Only when it keeps coming back even after a password was sent is there a problem with the login details.

Expiry time (Expires) and renewal

A registration does not last forever. Every registration has an expiry time, called Expires, measured in seconds. Common values are between one minute and one hour, and in many devices the default is 3600 seconds (one hour). The PBX can also shorten the time the phone asked for.

The phone does not wait for the registration to expire. It renews it a little earlier, usually halfway through or shortly before the end. As long as it is on and connected, it keeps renewing, and the extension stays registered.

Why is an expiry needed at all? Because devices disappear without notice: someone unplugs the phone, the computer with the softphone goes to sleep, a mobile phone leaves coverage. Without an expiry, the PBX would keep sending calls to an address where no one is anymore. The expiry cleans up the list by itself.

The router connection: why a registered extension does not ring

Here is one of the most common points of confusion. Registration tells the PBX where the phone is. But for an incoming call to actually reach it, the router in the office has to remember that this connection belongs to the phone.

Routers delete UDP connections that have no activity after a certain time. If the router deletes after 60 seconds, and the phone renews its registration once an hour, there are 59 minutes in which the PBX thinks the extension is registered — and the incoming call is dropped at the router. Outgoing calls work, incoming calls do not.

The solution is one of two: lengthen the hold time in the router, or make the phone send "I'm still here" messages (Keep-alive) more often. For this whole mechanism, see NAT.

Why an extension is "not registered" — the common causes

How to diagnose, step by step

When an extension is not registered, it is worth going in this order. It starts with the simple things and ends with the complex ones:

1. Is it only one extension? If the whole office is down — it is the internet or the router. If only one — it is that extension's device.
2. What does the display say? Most phones show an icon or a message when the account is not registered, and sometimes the reason too.
3. Restart — unplug the phone from power for ten seconds and plug it back in. Many momentary problems are solved this way.
4. The extension details — compare the username, the password and the server address with what appears in the control panel. A one-letter mistake is enough.
5. Another device on the same network — if a softphone on a mobile phone, on the office Wi-Fi, registers with the same details, the problem is in the phone. If it does not either — in the network.
6. A different network — if the softphone registers over mobile data but not on the office Wi-Fi, the problem is in the office router or firewall.

An extension that keeps dropping and re-registering

A particularly annoying situation: the extension is registered, after a few minutes it is not, and then registered again. The employee says that "sometimes calls don't come through." There are several known causes:

• A router that deletes connections quickly — as described above.
• SIP ALG — a setting in the router that changes the registration messages, sometimes incorrectly.
• Two routers in a chain — the provider's router and another office router, each translating addresses.
• Unstable Wi-Fi — in a softphone or a wireless phone that moves between access points.
• Two competing devices — both are registered with the same details, and in some PBXs every new registration "replaces" the previous one.

Our standard router recommendation — SIP ALG off and a UDP hold time of 180 seconds — handles the first two causes.

Registration and security

Registration is also the front door to the PBX. Anyone who has the username and password of an extension can register a device of their own and make calls at the organization's expense. So it is worth treating the extension details like a bank account password:

• A long, random password for each extension — not "1234" and not the extension number.
• Do not send the extension details in an open message or post them next to the phone.
• When an employee leaves — change the password of their extension.
• If possible, limit the addresses from which registration is allowed.
• Watch for an extension that is registered from an unexpected place.

More on this in toll fraud.

Several devices on one extension

Sometimes you want an extension to ring both on the desk phone and on a softphone on a mobile phone. There are two ways to do this, and what is supported depends on the PBX:

• Several registrations for the same extension — both devices register with the same details, and the PBX rings both. This works only if the PBX is set to allow more than one registration.
• A separate extension for each device — each device has its own extension, and a call to the main extension rings on both together, for example through a hunt group or Follow Me.

The second way is more orderly: you can see exactly which device answered, and if one of them disappears, the other keeps working without the "registrations" competing with each other.

An example from the field

At the "Keren Halvaah Hashchunah" gemach (a made-up name), the treasurer's extension, 102, appeared in the control panel as "registered," and yet calls to it almost never arrived. Outgoing calls from it worked fine.

The check showed a router that deletes UDP connections after 30 seconds, and a phone that renews its registration once an hour. For most of the hour, the PBX was trying to ring an address the router had already "forgotten." After the hold time was raised to 180 seconds and SIP ALG was turned off, calls started coming through. Again — no device was replaced.

Softphone and mobile: registration that comes and goes

In a softphone on a mobile phone, registration behaves differently from a desk phone. The mobile phone switches between Wi-Fi and mobile data, goes into battery-saving mode, and the operating system closes apps in the background. With each such switch its address changes, and the old registration is no longer correct.

Good apps deal with this in two ways: they re-register immediately when the network changes, and many of them use "push notifications" (Push) — a server that wakes the app when there is an incoming call, even when it is asleep. Without a mechanism like that, a softphone on a mobile phone may miss calls.

So when someone says "it doesn't always ring on my mobile," it is worth checking the phone's battery-saving settings first, and only then looking for a problem in the PBX.

What the PBX sees, and what it does not

From the PBX's point of view, a registered extension is a line in a list: the username, the address the registration came from, the type of device, and when the registration will expire. This is very useful information for diagnosis — for example, if the secretary's extension is registered from a home network address, someone probably set it up on a softphone at home too.

What the PBX does not see: whether the phone is still on right now. Until the registration expires, it assumes it is. So a phone that was unplugged five minutes ago can still show up as "registered," and a call to it will go to the destination set for no answer.