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A straightforward guide to SIP Trunking

A straightforward guide to SIP Trunking

A straightforward guide to SIP Trunking

What if you could operate a business phone system without any actual phone lines? That might sound like a contradiction in terms, but it's exactly what SIP trunks deliver. Through modern SIP trunking, you can extend your phone system beyond the confines of a conventional multi-phone line setup and enjoy much higher flexibility, scalability and savings by harnessing the power of IP networks.

SIP, explained

Before talking about trunks, we should explain SIP itself, as it’s what really sets trunking solutions apart. SIP stands for Session Initiation Protocol, which is one of the central protocols for enabling voice communications over data networks and providing an alternative to the public switched telephone network (PSTN).

Traditional voice service, via PSTN, is built on circuit-switched technology, whereby a physical connection is established between the different participants and the entire bandwidth of that circuit is held open for the duration of the call.

Packet-switching allows voice communications to instead be broken up into packets that flow over an IP network like the public internet, just like any other type of application traffic. SIP initiates the sessions necessary for voice, video and instant messaging on packet-switched networks. It's the core not only of SIP trunking, but also Voice over Internet Protocol (VoIP) and unified communications.

How SIP trunks work

A SIP trunk is not a tangible container or chest – on the contrary, it's a virtual alternative to analog phone lines. A SIP trunk foregoes a physical connection to the PSTN. Instead, it taps into existing network transport infrastructure, whether that's a T1/T3 line, an Ethernet-over-Copper, fiber or something else to connect to the SIP trunk provider's core network via a managed router with primary and backup circuits. The provider's core then interfaces with the internet and PSTN.

In this way, SIP trunks can consolidate voice and data traffic onto a common set of circuits or use VLANs to segment them for more predictable internet capacity. That means there aren't any physical phone lines to maintain, which, combined with the much lower cost of packet-switched voice service like VoIP compared with PSTN, yields substantial savings. An effective SIP trunking solution is a viable alternative to other multi-line setups such as key systems and private branch exchanges.

SIP trunking service in practice: How it can replace your PRI

Let's dive deeper into the cost efficiencies possible through SIP trunking by comparing it directly to physical connections to PSTN. The linchpin of traditional PSTN voice service is the Primary Rate Interface (PRI), each increment of which provides 23 voice channels and one signalling channel. But what if you don't need 23 new blocks when you're expanding your phone service?

Relying on PRI will often result in overbuying. Not only will you purchase more channels than you need, but you'll deal with the complexity of installing the new circuits and accompanying termination hardware – a process that can take days or weeks to complete. SIP provides a different and simpler way forward. Making a simple configuration change in software is usually all you will have to do to effectively scale your phone system.

At a more fundamental level, SIP is a virtual connection, unlike the physical connection of PRI. As such, it's not limited by hardware and cabling but only by the available network bandwidth, which is much easier to procure and manage.

Say your office contains 30 users who need phone access. With PRI, you would have to buy two blocks of 23, whereas with SIP you would have the liberty to purchase only the number of sessions necessary to handle peak call volume – which would be much lower even than the total number of employees in the office.

SIP trunking simplifies capacity planning and service expansion while reducing costs and complexity. Plus, with the right implementation it's easy to integrate into almost any phone system.

SIP trunking gives you choice and flexibility

Phone systems should meet your requirements and expectations, instead of placing constraints on how you operate. The best SIP trunking services give you options such as:

  • Routing your calls over the public internet or via private dedicated fiber.
  • Adjusting bandwidth based on expected and observed call volume.
  • Connecting via various handoffs like RJ-45/Ethernet, DS-1/PRI and DS-0/FXS.
  • Adding audio conferencing, caller ID, call waiting, E911 and other services.

Telesystem's SIP trunking solutions offer these capabilities within affordable plans backed by our expert team and portfolio of synergistic solutions. You can combine your SIP trunk with a dedicated internet connection or a hosted VoIP station to accelerate the switchover from your PBX to more scalable and economical voice service. We'll help you make a seamless transition that will free you from needing to purchase more PRI blocks or sink so much money into maintaining multiple analog phone lines.

To learn more, visit our SIP trunk page or take a look at our entire set of voice solutions. You can also contact our team directly or request a solution quote.


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