Insights & Resources9 min read

SIP Trunk vs DID - Direct Inward Dialing Basics

SIP Trunks and DIDs work together to make an efficient and cost effective calling solution.

Brian Segal
Dialing a DID number for SIP trunking
The comparison of a SIP trunk vs DID is an interesting one, because these aren’t really competing technologies or connection methods. A SIP trunk and a DID are actually two parts of a completed phone call that work together.
But it’s valuable to understand how they both work, because you’ll need to acquire and configure both of them to set up an operational phone system. Here’s what you need to know.

What is DID?

A DID (Direct Inward Dialing) number is a virtual phone number that connects to a specific phone, without dialing an extension or operator assistance. DIDs enable your employees to have a phone number that connects directly to their phone, without tons of physical phone lines.
If you have an office building with tens or hundreds of phones, you can assign a DID number to each phone, rather than using a few phone numbers and requiring callers to dial an extension to reach a specific phone in the building. This creates a far simpler and more enjoyable caller experience.
And the reason why SIP trunking and DID numbers aren’t competing technologies is that DIDs are phone numbers that work perfectly on a SIP trunk connection. Here’s a deeper dive on how it works:

How does Direct Inward Dialing Work?

A DID number connects over the internet, using a SIP trunk. The call path using a DID looks like this:
Caller > SIP trunk > Internet > PBX > Receiving PBX phone
If you only have one phone, you may not have a PBX in place (more on PBX systems in a moment), and calls would go straight from the internet to your phone. But you can still use a DID, even if you only have one phone.
This is a relatively simplified explanation though. There are details about each of these stages and the technologies involved at each stage.
  • SIP Trunk: A SIP (Session Initiation Protocol) trunk is the conduit that connects your phone to the internet. A SIP trunk is an alternative to traditional PRI (Primary Rate Interface) phone line trunks. SIP trunking is much easier to scale up and down than a PRI trunk, because you can add connections to a SIP trunk without adding physical wires.
  • VoIP (Voice Over Internet Protocol): VoIP is a protocol for transmitting phone calls over the internet. If you use a SIP trunk connection, you’ll almost always be using VoIP as your connection protocol. VoIP phone numbers are virtual phone numbers. But VoIP phone numbers look the same as traditional phone numbers. And the calling experience with a VoIP phone number is the same as the traditional calling experience.
  • VoIP Device: A VoIP device is simply a phone or other device that’s capable of making calls using VoIP phone numbers, without any adapters or special hardware. Smartphones, computers, and (obviously) VoIP phones are all VoIP devices.
  • PBX (Private Branch Exchange): A PBX system is a local phone system that connects many phones to your SIP trunk or PRI trunk. An office building with phones in many offices or cubicles uses a PBX system to connect all of these phones to the PRI trunk or SIP trunk and route calls to the right phone based on the phone number or extension.
  • PSTN (Public Switched Telephone Network): The PSTN is the traditional phone network that has existed for decades. The PSTN is a system of 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 and vice versa. A VoIP gateway decompresses and decodes digital signals, so that they can be transmitted over the PSTN. Conversely, a VoIP gateway will compress and encode analogue data for transmission over the internet. In short, a VoIP gateway is the 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 connect calls. But callers with traditional phone numbers that connect to the PSTN can still make calls to DID numbers and callers on VoIP devices can make calls to traditional phone numbers. In these situations, the phone data is processed through a VoIP gateway to make the connection between the internet and the PSTN.
That leads to another question: why use a DID rather than a traditional phone number?
The main benefit of using DID numbers over traditional phone numbers is that it’s much easier and more affordable to add more phones and phone numbers. Expanding traditional phone infrastructure often requires running new wires. On the other hand, you can add many DID numbers on a single internet connection, so long as you have the bandwidth to support calls from all your DIDs.
Here are a few popular uses cases for DID numbers, to add clarity:
  • PBX Systems: PBX systems have a lot of phones and require a lot of phone numbers to work. However, without DID numbers, callers would need to call a central business phone number, then dial an extension to route the call to the person they want. Or you’ll have to have an operator assist callers with reaching the person they want. Using DIDs enables you to assign a unique phone number to each phone. That way callers can simply dial a phone number to reach an employee in your office, even though that employee’s phone is connected to your PBX system.
  • Multi-Department Business Phones: Traditionally, the solution for having multiple departments was a touchtone or IVR menu. There are ways to make these menus as simple as possible. But using DIDs to give each department their own phone number that customers can call directly is far simpler and more enjoyable for customers. That way people don’t have to deal with any menus, they just connect directly to the department they need.
  • Communication Apps: Messenger and OTT communication apps and VoIP softphones can use DID numbers to produce a more familiar calling experience. That way, even though users are connecting through an app, it will be very similar—if not almost identical—to making a standard phone call.
  • Fax: You can connect fax machines to a PBX system or use them for taking orders or sending documents to certain departments. You can use a DID to give your fax machines a direct number, rather than using your central phone number with a fax extension. There are other potential uses for DID numbers. But using DIDs can be the best solution in any situation where you need to easily route calls to different phones.

SIP Trunk vs DID Providers

Your SIP trunk provider and your DID supplier may not be the same company. Most SIP trunking providers also offer DID numbers. But you could use one provider for your SIP trunk connection and another provider for your DIDs.
If you’re making a SIP trunk vs DID comparison, you’re really considering the cost of using a separate SIP trunking provider and DID supplier and getting your SIP connections and DID numbers from the same provider. The short answer to this question is that it’s almost always simpler and more cost efficient to get your DID numbers from your SIP trunking provider.
Here’s how your DIDs and SIP trunks work together.

How DID Numbers Work with SIP Trunk

In short, your DID number identifies a specific phone. Your SIP trunk is the connection between that phone and the internet. When someone calls a DID, the SIP trunk connects the call to the internet. And the DID enables that call to be routed to the right phone.
Since a DID is a virtual number, it can be assigned to any phone or device. And DIDs are easy to reassign to new phones and devices. And you can have an almost unlimited number of DID numbers on a single SIP trunk.
The only limitation is the bandwidth of your internet connection, and even that doesn’t limit the potential volume of DID numbers you can have on your SIP trunk. Your bandwidth only limits the number of simultaneous calls you can make.
This is why it’s almost always simpler and more cost efficient to get your DID numbers from the same provider that you get your SIP trunking connection from. Otherwise you may end up paying a slightly higher price for each DID you need, and that slightly higher price can add up quickly when you purchase a whole lot of phone numbers.
DID numbers coupled with SIP trunking make a cost efficient solution that produces high quality calls. But there is another solution.

DID vs Extension Numbers

When you use DID numbers, you get a unique phone number for each phone you have. Callers simply dial the number, and the call gets routed to the associated phone.
The alternative is extension numbers. Extension numbers work kind of like the old phone system where the operator manually routed calls. Extension numbers just streamline this process.
With extension numbers, you’ll have just a single phone number or a few phone numbers. These phone numbers will connect to a central routing hub. Each phone connected to that routing hub will have an associated extension number, which is usually 3 or 4 digits.
When someone calls one of your phone numbers, they’ll be prompted to enter the extension that connects to the phone they’d like to reach. If they don’t know the extension, they’ll usually get connected to a customer service representative that can help them and route the call.
This system works. But it’s certainly not as efficient as having a phone number that connects directly to each phone, so that callers never need to dial an extension or talk to a customer service representative to reach the person they want.
Using DID numbers makes the experience more enjoyable for callers and reduces the resources you must invest in your telephone infrastructure.

Where to Purchase DID Numbers

The simple answer here is that you get DID numbers from VoIP DID providers. The more complex part of this process is figuring out which VoIP DID providers are best. There are a few key features that you should look for in a VoIP DID provider.
First, it’s best if your DID provider also provides your SIP trunk connection. This is usually more affordable and it simplifies your phone infrastructure and reduces how many invoices you must handle each month.
Additionally, your DID and SIP trunking provider should own and operate their own IP (Internet Protocol) network. That way your provider can easily troubleshoot connection problems on their network. A private network will also perform much better and be more secure than relying on the public internet to connect calls.
So, when you’re comparing all the potential VoIP DID providers, look for ones that also offer SIP trunking and have their own network. That way you’ll get the most affordable DIDs and the best call quality and reliability.
Looking for a VoIP DID provider that passes both of these tests with flying colors? Connect with a Telnyx expert and find out how easy it is to get DID numbers through the Telnyx Mission Control Portal (hint: it only takes a few clicks). Or, follow our self-service guide to get started with SIP Trunking and set up your voice communications with carrier-grade quality and global reach.
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