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Documentation › Phone system

Extensions and numbering

Give every person, room and service in your business a short number of its own — and let Filantis keep the numbering tidy as you grow.

Last updated 19 September 2026

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At a glance

  • An extension is a short internal number — 101 or 1001 — that rings a person, a room or a service.
  • Calls between extensions are free, whether the phones are on one desk or in two cities.
  • You choose 3-digit or 4-digit numbering once, when you create your workspace.
  • Filantis hands out the next free extension for you and refuses one that would clash.
  • One extension can ring many phones — a desk phone, the mobile app and the browser at once.

On this page:

  1. What an extension is
  2. 3-digit or 4-digit
  3. Your numbering plan
  4. Give someone an extension
  5. Voicemail, forwarding and DND
  6. Phones, the app and the browser
  7. Move or free an extension
  8. Extensions that are not people
  9. Change the ranges
  10. Questions

1What an extension is

Your business has one or more phone numbers that the outside world dials. Inside, every person and place gets a short extension instead. Colleagues dial each other's extension, calls from the outside world are routed to one, and nobody has to remember a ten-digit number to reach the kitchen.

Filantis keeps three things apart, so each can change without disturbing the others:

The employee A person on your team, with a login to Filantis. Not everyone needs a phone.

1001

The extension The number that rings them, with its own voicemail and forwarding rules.
The devices Desk phone, mobile app, browser — each with its own sign-in.

A housekeeper, a kitchen porter or a driver can be on Employees for attendance and payroll without ever having an extension. You only use an extension on the people who take calls.

23-digit or 4-digit numbering

When you create your workspace, Filantis asks how long your extensions should be. Everything else — call menus, meeting rooms, park slots — is numbered to match.

101

3-digit

Employees from 100 to 499. Quick to dial and easy to remember — ideal for a shop, a clinic or a small office.

Every plan

1001

4-digit

Employees from 1000 to 4999. Room to grow, and what most offices and hotels expect.

Medium and Large sizes

Choose once. The length is set when the workspace is created and cannot be switched later, because every phone, call menu and forwarding rule is built on it. If you run a hotel with rooms above 999, choose 4-digit.

3Your numbering plan

Each kind of extension has its own block of numbers, so the first digit tells you what an extension is — in both 3-digit and 4-digit workspaces.

Read an extension by its first digit. 1004 is a person, 6001 a call menu, 7001 a parked call.

These are the blocks every new workspace starts with:

Used for3-digit4-digitGiven out on
Employees
People on your team		 100–499 1000–4999 Extensions
Call attendants
Voice menus: “press 1 for sales”		 600–649 6000–6499 Call Attendants
Auto dialer
Outbound calling campaigns		 650–659 6500–6599 Auto Dialer
Park slots
Put a call on hold for someone else to collect		 700–719 7000–7099 Parking Lot
Meeting rooms
Conference calls, with an optional PIN		 800–819 8000–8099 Meeting Rooms
Paging and intercom
Paging groups and speakers		 820–839 8100–8199 Paging/Intercom
DISA access
Dial in from outside, then dial out		 840–849 8200–8299 DISA System
Service extensions
Set aside for your own special numbers		 500–549 5000–5099 —
Fax
Fax machines and fax adapters		 990–999 9990–9999 Printers & Fax
Reserved
Held back for future features		 900–979 9000–9089 —

You never have to track which numbers are taken. Every page that hands out an extension offers the next free one in the right block, and turns away one that belongs to something else — with the reason, so you know where to look.

4Give someone an extension

You need the admin role, and the person already on Office → Employees. You can also give an extension while adding the employee — it is optional on that form.

1Add the employeeOffice → Employees
2Assign an extensionExtensions page → Assign
3Sign in a phoneDesk phone, app QR code or browser
4It ringsCalls, voicemail and forwarding work
From new starter to ringing phone — this section covers step 2.
  1. Open Phone & Extensions → Extensions. The bar above the list shows your numbering plan and the next free extension.
  2. Click Assign an extension.
  3. In Who is it for, pick the employee. Only people without an extension are listed.
  4. Keep the suggested extension, or type another inside the Employees block.
  5. Click Assign. The extension appears in the list, ready for a phone to sign in.

Your plan sets how many extensions you can have: 10 on Mini, 60 on Small, 100 on Medium, 140 on Large. Need more? Move up a size at any time — nothing is renumbered.

5Voicemail, forwarding and Do Not Disturb

Click the pencil on an extension's row to decide what happens to its calls.

Voicemail

Tick Enable voicemail and set a 4-digit Voicemail PIN. Unread messages show as a count on the extension's row — click it to listen — and in the app.

Call Transfer

Set how long the extension rings with Ring for (30 seconds unless you change it), then choose where a call goes when it is not answered, when the line is busy, or always. A call can go to another employee, a department, a voicemail box, a call attendant, an outside number, or a recorded message.

The Default tab applies to everyone's calls. Add rules on the Available, Away, Busy or DND tabs to behave differently when the employee sets that status — for example, straight to a colleague when they are Away.

Do Not Disturb and the whitelist

When an employee sets their status to DND, the rules on the DND tab decide where their calls go — straight to voicemail, for example. Put extensions in FWD Whitelist — a manager's, say — and their calls always ring through, whatever the rules say.

How a call finds its way

Flow chart: what happens to a call for extension 1001 A call for 1001 Is the caller on the FWD Whitelist? YES Rings 1001 directly forwarding rules are skipped NO Is a Forward Always rule set for their status? YES Goes to that destination 1001 does not ring NO Rings every device together for the Ring for time (30 s) Answered the call is connected Busy already on a call Not answered or every phone is off Forward Busy rule? set: goes to its destination Forward No Answer rule? set: goes to its destination NOT SET NOT SET The caller reaches 1001's voicemail when voicemail is switched on for the extension A check Filantis makes What happens next Call connected
Rules for the employee's current status are checked first, then the Default tab.

A phone that is switched off or out of signal counts as not answered, so your no-answer rule catches it. Nobody reaches a dead end.

6Phones, the app and the browser

One extension can ring several devices together. Each gets its own sign-in, so a lost phone is switched off on its own without changing anyone's password.

Extension 1001 ringing a desk phone, the mobile app and the browser together 1001 one extension Desk phone The Devices page, or Phone Provisioning Mobile app Android — scan a QR code to sign in Browser The dial pad inside Filantis — nothing to install
All ring together. Answer on whichever is closest.
  • Desk phones — add them under Phone & Extensions → Devices, or let Phone Provisioning set them up for you — see Set up desk phones.
  • Mobile app — the employee scans a QR code and the app signs in to their extension (on plans that include the app) — see Install the mobile app.
  • Browser — the dial pad inside Filantis rings with no install at all.

The dot beside each extension shows its live state:

  • No device signed in
  • Available
  • Away
  • On a call or busy
  • Do Not Disturb

7Move or free an extension

To give an extension to someone else, or to renumber a person:

1004

Priya Sharmamoves to the Mumbai branch
FreeThe bin on the row — Priya stays an employee

1004

Back in the poolOffered as the next free extension

1004

Arjun MehtaAssigned — his phones sign in
An extension belongs to the business, so it can pass from one person to the next.
  1. On Extensions, click the bin on the row — Free this extension. The number is released; the employee stays.
  2. Click Assign an extension and give the number to its new owner, or a new number to the same person.

The bin on Extensions frees a number. The bin on Employees removes the person. When somebody leaves, free their extension first if you want to reuse it straight away.

8Extensions that are not people

Some of the numbers your business dials are places and services. Each is numbered on the page where you set it up:

  • Office zones — a handset on the back gate or a paging speaker in the lobby. Give them an extension on Office → Office Zones. They also appear, read-only, at the foot of the Extensions page, so it always answers “who is 8112?”.
  • Hotel rooms — room phones are numbered from the room number when you lay out the floor plan, so staff dial 204 — or a pattern such as 9204 — to reach room 204.
  • Call attendants, meeting rooms and park slots — numbered from their blocks when you create them.

Departments have no extension. You route calls to a department by choosing it — in a call menu, a forwarding rule or an incoming-number route — and Filantis rings its members.

6001

Call menu“For reservations, press 2”
ReservationsA department — chosen by name, no extension needed
Its members ring1004 Priya · 1005 Arjun · 1009 Meera
Staff change, the route doesn't. Add someone to the department and they start taking its calls.

9Change the ranges

The blocks suit most businesses as they are. To widen or move one, open Settings → Extension Scheme, click Edit on a range, change its Start and End, and save.

If anything ever looks out of step, Re-check on the Extensions page rebuilds the list from every page that hands out extensions.

Blocks must not overlap, and must keep the length of your scheme — a 3-digit workspace cannot have a range that reaches 1000.

10Questions

Do calls between extensions cost anything?

No. Calls between extensions never leave Filantis, so they are free on every plan — between desks, between branches, or to a colleague on the mobile app abroad.

Can I keep the extension numbers we use today?

Yes, as long as they fit your scheme. Type the number you want when you assign it instead of taking the suggestion.

Can I switch from 3-digit to 4-digit later?

Not from the admin pages — the length is fixed when the workspace is created. If you have outgrown 3-digit numbering, write to us and we will help you plan the move.

Does every employee need an extension?

No. Only people who take calls need one. Everyone else can still use attendance, payroll and the rest of Filantis.

What is the difference between an extension and a phone number?

A phone number (a DID) comes from your carrier and is what customers dial. An extension is internal. You point each phone number at an extension, a department or a call menu.

One number plan for the whole business

Filantis is your phone system, staff register and accounts in one place — so the extension that rings the front desk is the same one on the payroll, the attendance sheet and the guest's bill.

  • No phone box to buyRuns in our cloud, or on a server in your own office.
  • Any phoneDesk phones, Android, Windows and the browser.
  • Keep your carrierBring the SIP trunk and numbers you already have.
Create my free workspace See the phone system

What to do next

With your extensions in place, build the front door that callers hear: a call menu that sends them to the right person. Read Build a call menu (IVR) next, then Departments and ring groups.

Still stuck?

Write to hello@filantis.com with your workspace name and what you were trying to do.