Understanding DIDs and virtual numbers

What a DID is

A DID is a phone number that routes calls directly to a destination on your phone system, such as an extension, a team, a queue or a menu, without passing through an operator or a main switchboard. Callers dial it like any other number; your phone system decides where the call goes.

The term comes from Direct Inward Dialling, a feature of traditional business phone systems. A carrier would assign a block of numbers to a company’s trunk lines and, for each incoming call, pass the dialled digits to the PBX so it could ring the right extension. Before this, an outside caller reached a main number and an operator connected them.

With SIP trunking, the principle is unchanged but the delivery is different. Numbers are no longer tied to physical lines into a building. The carrier routes each number to a SIP trunk, and the trunk delivers the call over IP to wherever your PBX runs.

DID, DDI and virtual numbers

Several terms describe much the same thing, depending on region and context.

• DID (Direct Inward Dialling) is the usual term in North America and in much of the VoIP industry.
• DDI (Direct Dial-In) is the usual term in the UK and in many other countries. A UK business with a range of DDIs has the same capability as a North American business with a block of DIDs.
• Virtual number emphasises that the number is not tied to a physical line or location. It can be answered by a PBX in another city, a cloud phone system or forwarded to another number.

In practice, providers use these terms interchangeably. What matters is the number type, the country it belongs to and the rules that come with it.

Number types

Most countries divide their numbering plan into types with different purposes, costs to callers and regulatory requirements. Names and availability vary by country, and not every type can be provided for VoIP use everywhere.

As an example from one country, UK geographic numbers begin with 01 or 02, national-rate non-geographic numbers with 03, mobile numbers with 07 and freephone numbers with 0800 or 0808. In North America, toll-free numbers use codes such as 800, 888 and 877. Other countries have their own structures, so check the numbering plan for each market you need.

How inbound routing works

When someone dials one of your numbers, the call passes through four stages.

1. The caller’s network recognises the number and hands the call to the carrier that holds it.
2. The carrier looks up where that number is assigned and routes the call to your provider’s platform.
3. The SIP trunk delivers the call to your PBX or cloud phone system, presenting the dialled number in an agreed format.
4. Your PBX matches the dialled number against its inbound rules and sends the call to the configured destination: an extension, ring group, queue, IVR menu or time-based rule.

Because the routing decision in the final stage belongs to your phone system, numbers can be reassigned between people, teams or sites without any change at the carrier. A provider may also be able to apply failover, sending calls for a number to an alternative destination if your system cannot be reached; this is agreed and tested in advance.

One trunk can carry many numbers. The number of calls that can be in progress at once is set by the trunk’s channels, not by how many numbers you hold. See How to estimate SIP channel requirements for how channels are sized.

Number formats and E.164

E.164 is the international standard for telephone numbers, published by the ITU. An E.164 number consists of a country code followed by the national number without its leading trunk prefix, and is limited to fifteen digits in total. It is usually written with a leading plus sign.

Using E.164 consistently between your PBX and your provider avoids a common class of faults: inbound routes that do not match because the number arrives in a different format, and outbound calls rejected because the number was sent in national format. Agree the format with your provider and configure the PBX to match.

Regulatory documentation

Telecoms regulators in many countries require providers to know who is using each number. Before a number can be activated, you may be asked for some or all of the following:

• The legal name of the business or individual who will use the number.
• Business registration details, such as a company registration number or certificate.
• Proof of address, sometimes required to be within the area that a geographic number serves.
• Proof of local presence, where a country restricts certain numbers to residents or locally registered businesses.
• Identification for the authorised person signing the order.

Requirements differ by country and number type, and regulators update them from time to time. Some numbers can be activated quickly once documents are approved; others involve a longer review. Gather documents early, particularly for countries where you have no office.

Porting existing numbers

Number porting moves a number you already use from one provider to another, so callers keep dialling the same number. Whether a number can be ported depends on the country, the number type and the providers involved.

1. Check portability first. Your new provider should confirm that each number can be ported before you sign anything or give notice to your current provider.
2. Sign a letter of authorisation. The LOA gives the new provider permission to request the port on your behalf. It is signed by the account holder or someone authorised to act for them.
3. Match the details exactly. The account name, address, account number and number list on the request must match the current provider’s records. Mismatches are the most common reason a port is rejected.
4. Agree the port date. Timelines are set by the carriers involved and by each country’s porting process. They vary considerably, and multi-number or multi-site ports usually take longer.
5. Prepare the new routing. Configure inbound routes for the numbers on the new trunk before the port date so calls are answered as soon as the switch happens.
6. Test after the port. Call each ported number from an outside line and confirm it reaches the right destination.

Do not cancel your existing service before the port has completed. In many cases cancelling the old account releases the numbers, and they may be lost permanently. Where a number belongs to a range or is linked to a main billing number, ask how the rest of the range is affected.

Caller ID presentation

Caller ID is the number shown to the person you call. With a SIP trunk, your PBX sets it on each outbound call, so it can present your main number, a department number or a user’s direct number.

You may only present numbers that are assigned to you or that you have permission to use. Regulators in many countries require presented numbers to be valid and able to receive calls, and providers commonly reject or replace caller ID that does not match a number on the account. Some countries add further measures; in the United States and Canada, for example, the STIR/SHAKEN framework lets carriers attest to whether a caller is entitled to the number shown.

Send caller ID in E.164 format unless your provider specifies otherwise, and test it by calling a mobile and checking what appears. If you need to present a number held with another provider, ask what proof of authorisation is required.

What affects cost

Number costs are rarely a single figure. The main factors are:

• Country and number type. Monthly charges differ widely between countries and between geographic, national and toll-free numbers.
• Inbound usage. For toll-free numbers the recipient pays for inbound calls, often per minute and varying by the caller’s network, such as landline or mobile.
• Set-up and regulatory checks. Some numbers carry one-off activation or verification charges.
• Porting. Some providers charge to port numbers in or out, and complex ports may involve additional work.
• Channels. The capacity to carry calls is usually priced separately from the numbers themselves.

When comparing quotes, check that each line is itemised so you can see what is charged per number, per minute and per channel.