Servers 101: What Types Of It Are Used for SIP?

If you’ve ever used internet-reliant communication, congratulations! You’ve just become a part of the business internet age and have enjoyed the following benefits:

  • Smaller costs for communication.

  • Ability to reach people, no matter their location, and at no exorbitant costs.

  • Be free from outdated hardware that’s very expensive to maintain.

  • And many more!


That’s why companies that offer such services for buy virtual phone number like are increasing. Because the more that people learn about these benefits, the more enamored they become to make the switch.

One of the more popular technology options is SIP. This post would focus on the servers it uses. But to get everyone up to speed, we’ll have a quick run of what SIP is.

A Quick Overview of SIP


Without going overly technical on this, SIP is something like a foundation - it’s a hundred percent necessary for whatever it is you’re building, but it’s not sufficient on its own.

And like how you can’t use the foundation for a determined end-function, you can’t use SIP to make internet calls. But you need it to make the call happen.

Built long before internet-reliant communication gained traction, SIP is actually the standard right now. It’s what other protocols are built upon. So despite being relatively old, it never becomes obsolete.

What SIP does is:

  • Detects user “location” based on the devices they registered.

  • Validates the availability of other users.

  • Set and activate features to allow different communication modes like text messaging and video conferencing.

  • Send signals in real-time so you can see the latest notifications and requests to connect.

Servers that SIP Uses


Proxy server

This is probably the server that you’re most familiar with. The main thing that you should know about this server is that it receives requests and sends them to their destination. That’s it.

It is essentially like a postal service, only operating at unimaginable speeds per second. Its only concern is the packet header, which is like the address of sorts, determining where the packet will be routed.

There are two types of this that you should know about:

1. Stateless

Of the two, it’s the more basic type because it only does one thing: process each of the SIP packets that go through it. It doesn’t retain any information about the call, such as where it’s going, the content of the message, or even the message source. 

And although basic, what’s great about this is it can process huge chunks of data quickly and efficiently.

2. Stateful

Unlike its counterpart, a stateful proxy retains information so it would be able to monitor the state of each message and perform any action based on that information.

For example, if the message didn’t reach its receiver, the SIP packet will be sent again.

Redirect server

While the proxy server passes on information to another of its kind, the redirect server doesn’t. However, they’re the same in that they both respond to SIP requests.

This is mostly used when forwarding calls to another department or employee normally happens. 

Back to Back User Agent (B2BUA) server

Apart from the proxy server, one of the more common types is the B2BUA. But this type does more than receive requests. It also processes whatever media information it receives.

There are two types that you should know of:

1. PBX

We’re sure you’ve heard of this because it’s a very common solution introduced to businesses that need help managing calls from their customers.

That’s why you’ll normally associate this with call centers as they handle massive volumes of calls, which can become messy without any help from existing technologies.

2. Special border controller

Think of this as a super upgraded B2BUA. Apart from processing media information, it also comes with features that a lot of businesses find valuable such as:

  • Encryption of data to protect your privacy

  • Repair of the protocol to fix any issues with SIP implementation

  • Define resource access by having control over who can get to them

Why should you know about this?


Think of SIP calling as food. There’s more than one way that you can eat your food and get your body to absorb nutrients. You can chew solid food, you can slurp up a smoothie, or you can even get nutrition directly to your bloodstream. Of course, each has its own advantages and disadvantages.

In some sense, it’s the same way with SIP servers.

While the end result is the same, there are many ways to get there. Having sufficient knowledge on this allows you to:

  • Feel more confident about your decision to try SIP calling.

  • Ask questions that are more useful to your business when inquiring about the service.

  • Decide based on your knowledge instead of just relying on service providers.

And the best way to start with that is by learning about the different server types SIP uses.