If you don't know what SIP is, click here to see our previous article explaining the technology.
Basically, SIP is a technology used to deliver communication. When we talk about a SIP Trunk, it usually implies that a carrier, like AT&T, is delivering phone services to you over an IP connection using the SIP protocol.
This is different from legacy technologies like T1's or E1's which delivered phone services exclusively over a dedicated circuit. These circuits were not IP based, and would not work with systems that were not specifically designed to interface with the T1 or E1.
Once modern phone systems started to realize it's way cheaper to share the same network that computers used instead of having their own separate phone network, technology started to innovate in that direction. Eventually, SIP Trunking was born.
A SIP Trunk delivers voice services over any IP circuit. That means you can get a SIP trunk directly over the Internet if you wanted – though we would not recommend this. Because voice traffic is really sensitive to quality issues, putting it on the Internet means you're opening yourself up to problems that you just can't fix. The Internet is like a small child – sometimes it's well behaved, other times it throws tantrums and it feels like there is nothing you can do about it.
Because the Internet is intermittent garbage, we suggest all SIP Trunking services are delivered over a dedicated circuit where you can apply quality control, or Quality of Service, tagging to the trunk. This will ensure that your voice service quality stays in tip-top shape.
SIP Trunks are usually sold in “channels”. A channel being a phone call. If you need to support 50 simultaneous phone calls, you'd need 50 channels. This again is different from the T1/E1 days, where you were pretty much forced to buy blocks of 23 or 30 channels depending on the technology used. SIP trunks can be bought for the specific number you need – the only requirement for capacity is bandwidth on the circuit.
Another benefit of SIP Trunking is the additional features you get when compared to T1's. Because it's delivered over IP, you're no longer physically tied to each location. With T1's, when that circuit went down so did every single phone number attached to it. With SIP, if a SIP Trunk goes down you can dynamically have all numbers re-route to another location. It also allows you to easily centralize your communications. Since you don't need to have a T1 at every location, you can save money in the long run by having redundant SIP trunks in your data-centers delivering service to your organization.
So that's SIP Trunking. It's pretty much the preferred method for delivering telephone service today.
If you want to know how SIP works, I invite you to take my Udemy Course: Introduction to SIP! Click here to go there now.