Xeta Group
(Session Initiation Protocol Trunk)
Making the move from PSTN to SIP Trunk: SIP Trunking explained
The trusted old Public Switched Telephone Network (PSTN), with its Analog lines, ISDN BRI, E1 or T1 lines, are gradually being replaced by IP telephony. An increasing number of businesses are using more modern and flexible SIP Trunks.

The major telecom providers are fast phasing out the old PSTN functionality, and are moving customers to IP. The majority of businesses these days are already using modern IP phone systems and SIP trunks, although some smaller businesses may still rely on outdated technology.

Businesses today are constantly looking for ways to cut costs and reprioritize your spending. Maybe you've heard of SIP trunking, and maybe you know other businesses saving money with SIP trunks, but you'd like to know more about how it works before you contact providers.

What Is SIP Trunking And How Does It Work?

What Is A SIP Trunk?
A SIP trunk
is the virtual version of an analog phone line. A SIP Trunk is the emerging standard for VoIP trunk service using Session Initiation Protocol (SIP) to set-up voice calls over public or private IP networks. SIP trunking service is used by businesses to provide local and long distance voice service as well as other applications. SIP trunk service provides increased efficiency through an IP PBX phone system by utilizing IP network access for SIP voice calls, without the need for regular phone lines.

SIP VoIP service is very flexible and can work with numerous protocols and scale quickly. SIP trunking providers also offer non-native variations of SIP VoIP service including SIP PRI and SIP T1 handoffs. SIP VoIP services are also known as SIP Trunk Services, SIP Trunking Services, SIP Lines, VoIP PRI, VoIP T1, Hosted VoIP PBX, and Hosted IP PBX.

How SIP Trunking Works.
A SIP trunk is the modern day equivalent of a T1 trunk. Where in the past you would buy a T1 trunk from a Telco provider and connect it to your legacy PBX, today you can buy a SIP trunk from an ITSP (Internet Telephony Service Provider) and connect it to your VoIP/IP PBX. SIP trunks use the SIP standard. The name 'trunk' comes from the telecom world and it means, more or less, a group of phone lines.

Using this standard protocol, telecom service (VoIP) providers connect one or more channels to your PBX, allowing you to make local, long distance, and international calls over the Internet. Phone numbers and DIDs are linked to the SIP trunk and in most cases numbers can be ported to the SIP Trunk. If you have an on-premises PBX in your office, a SIP trunk provider can connect to you and allow you to make outbound calls on your existing system, without restrictions on the number of concurrent calls.
What Is SIP Trunking And How It Works
How SIP Trunking Works

SIP Trunking For Dummies Download
SIP Trunking For Dummies
Learn To:
  • Understand The Basic Of SIP Trunking
  • Realize Huge Cost Savings With SIP Trunking
  • Appreciate The Importance Of An SBC In SIP Trunking
  • Choose A SIP Trunking Solution That Is Future Proof
Benefits Of SIP Trunking
Our farewell to the PSTN brings many benefits. SIP Trunks deliver:
Lower Monthly Line & DID Rental
The monthly fee to have a number of lines installed at your office drops significantly with SIP Trunks and can be as much as 30-40% less than PRI. And DIDs cost a lot less.
Lower Call Charges
There are many SIP Trunk providers and competition has driven down call charges significantly. Some SIP Trunks even come with unlimited calling and you can save on international call charges, too.
Only Pay For Capacity Needed
Sold by vendors on a per-channel basis on-demand, so you only pay for the capacity needed.
Disaster Recovery
Offers failover to mobile phones in case of loss of office data connectivity
Flexibility And Scalability
Offers simple support for multiple sites and remote workers. It is easy to add channels to your SIP Trunk to cope with increased calls. A simple phone call will allow you to add channels, and often this can be done immediately. Some providers include easy-to-use administrative portals for easy management. Compare that to the delay in having additional lines installed and then having to upgrade your old PBX to handle more lines!
Move Offices And Keep The Same Number
SIP Trunks are not bound to a location, so it's easy to move offices without having to change your stationary or inform your customers. There is no longer any need to pay to forward phone calls to the new offices.
Eliminate VoIP Gateways
SIP Trunks will eliminate the need to buy and manage VoIP Gateways. All phone calls come in via IP. No extra conversion often means better quality too.
Leverage A Modern IP PBX
Modern IP PBX / Unified Communications solutions will give customers increased productivity, mobility and boost sales. Can be integrated with multimedia communications for collaboration and productivity. May include rich mobile features and mobile-first design. Connecting an IP PBX to SIP Trunks is much easier than via the PSTN. You can go on-premise or hosted, the choice is yours.
PRI Hand-off
If your PBX does accept native SIP then some providers offer a PRI hand-off to the PBX
Selecting The Right SIP Trunk Service Provider
The next step is to choose a SIP Trunk provider who will supply the necessary SIP Trunks:
As SIP Trunks are exposed to the Internet, it is very important that the SIP Trunk has a well secured network and an anti-fraud system in place. The anti fraud system must monitor the system and provide protection against call fraud.
Own Network
Does the SIP Trunk provider run its own network or is it a rebranded service? There are quite a few providers out there reselling SIP Trunks from other providers. Select a provider who has control over their service and network.
Competitive Cost
Costs vary widely between services. Some vendors will overcharge for SIP Trunks. Look for competitive rates, but ensure that you are getting business quality SIP Trunk service. For example, telecom providers will provide a cheaper quality to Internet call shops. Be cost-conscious, but expect to pay a bit more for business class service.
Number Porting
Can the provider port your phone numbers? Ensure that you choose a provider who can port all the existing numbers – not all providers are able to do this for all regions.
Does the provider offer support? And what is their availability? Xeta Group has a wide range of vetted SIP trunk providers.
Upgrading Internet Connectivity
Once you have selected your SIP Trunk provider, consider a dedicated Internet line for the SIP Trunk. Most firewalls are able to handle multiple WAN connections, and, considering the low cost of an Internet line in most places, a separate VoIP connection will be the most reliable way to ensure the quality of your VoIP calls.

However, some SIP Trunk providers bundle their service with a dedicated Internet line. This keeps your voice traffic separate from your data traffic. Much will depend on the cost and your network infrastructure. Check that your firewall is up-to-date and will be capable of handling VoIP traffic correctly.
Upgrading The PBX To An IP PBX
Chances are that the old PSTN lines are connected into another old device, the hardware-based PBX system. This device is inflexible, difficult to manage and often expensive to maintain. Technically it is possible to buy a gateway that allows the old PBX to talk to the SIP Trunks. But why not upgrade to a modern IP PBX and leverage the flexibility and modern features IP telephony can bring to your business phone system. This allows you to take advantage of the cost savings, easy management, and productivity increases with full-scale Unified Communications that an IP PBX offers.
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