SIP • Last Updated 9/29/2023

What is Direct Inward Dialing?

Direct Inward Dialing (DID) is a service that enables businesses to receive inbound calls without extensions or operators.

Inbound Call Center Employees

Direct Inward Dialing (DID) is a service that enables businesses to receive inbound calls without extensions or operators. It supplies companies with virtual numbers (also called DID numbers) that function as regular phone numbers; however, DIDs are not connected to any landline. Cost-efficient and easy to use, DID—when paired with SIP trunking—eliminates the need to build physical phone lines.

How does Direct Inward Dialing work?

A DID number initiates calls over the internet using a SIP trunk. The call path using a DID number looks like this:

DID number > SIP trunk > Internet > Private Branch Exchange (PBX) > Receiving PBX phone

If you only have one phone, you may not have a PBX; calls would route from the internet directly to your phone. However, you can still use DID even if you have one phone. This is a relatively simplified explanation though. Let’s take a deeper look at each of these technologies and how they interact:

  • SIP Trunk: A SIP (Session Initiation Protocol) trunk is the conduit that connects your phone to the internet. SIP trunking is an alternative to traditional Primary Rate Interface (PRI) phone line trunks and is easier to scale than a PRI trunk; you can add connections to a SIP trunk without adding physical wires.
  • VoIP: VoIP (Voice over Internet Protocol) is a protocol for transmitting phone calls over the internet. If you use a SIP trunk connection, you’ll almost always use VoIP as your connection protocol. VoIP phone numbers are virtual but they look and function the same as traditional phone numbers.
  • VoIP Device: A VoIP device makes calls over the internet. VoIP devices use virtual numbers, without any adapters or special hardware. Examples of VoIP devices include smartphones, computers, and VoIP phones.
  • Private Branch Exchange (PBX): A PBX system is a local phone system that connects many phones to a SIP or PRI trunk. An office building with multiple phones uses a PBX system to connect phones to a PRI or SIP trunk. Calls are routed to the appropriate line based on the associated phone or extension number.
  • Public Switched Telephone Network (PSTN): The PSTN is a traditional phone network that has existed for decades. It operates on copper phone wires and switches that route calls from one phone to another.
  • VoIP Gateway: A VoIP gateway connects VoIP phone numbers and devices to the PSTN by decompressing and decoding digital signals. It compresses and encodes analog data for transmission over the internet, thereby facilitating a connection between the internet and the PSTN.

To put all of this together, a DID is a virtual phone number that uses SIP trunking and the internet to make calls. Callers with traditional phone numbers that connect to the PSTN can still call DID numbers. Conversely, callers using VoIP devices can make calls to traditional phone numbers. Phone data is processed through a VoIP gateway to connect the internet and the PSTN.

Get DID numbers on demand

Looking for a DID provider that offers SIP trunk connections? Telnyx streamlines your VoIP operations by providing both DID numbers and SIP trunking. This is more affordable, simplifies your phone infrastructure and reduces how many invoices you manage each month. We also own and operate our own IP, so your calls bypass congestion on the public internet, making them more secure and reliable.

Get on-demand phone numbers with carrier-grade quality and global reach. Contact a member of our team today.

By Michelle Bonsu

Michelle is one of our product marketing managers here at Telnyx.

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