This content is part of the Essential Guide: The basics of SIP trunking explained

Finding SIP trunking providers that meet your needs

Part two of our series, SIP Trunking Explained, offers advice on how to find SIP trunking providers that meet your needs and how to enable your legacy equipment to make SIP trunking services possible.

Editor's note:  Part two of our SIP Trunking Explained series looks at how to select a SIP trunking provider and enabling SIP trunking in your legacy equipment. Check out the rest of the series (see box below) for essential information on SIP vs. PRI, calculating how much bandwidth you'll need for SIP trunking, the SIP trunking advantages for VoIP over circuit-switched options and the security implications of SIP trunking on your network.

For organizations that have decided SIP trunking is in their future to provide telephony and unified communications services, the next step is to try a service offered by a SIP trunking provider to discover how well it works for them and how much they can save by switching.

SIP trunking has been a realistic option for businesses for the past three to four years in Europe and for the past five to seven years in the U.S.  Its acceptance as a viable option is constantly growing, with rapid growth in the past three years. One of the most frequent questions businesses ask is whether their existing private branch exchange (PBX) telephony systems already support SIP trunking. Finding out shouldn't be difficult; some quick research on the PBX vendor and model should quickly provide the answer.

In most cases, telephony systems are modular and allow owners to extend their functionality by purchasing the appropriate interface cards. Adding a SIP trunk could be as easy as adding the right card to your existing telephony system or upgrading the system's software and purchasing a SIP trunking license. In terms of licensing, SIP trunking is now supported by most vendors. Whether you need a license to use SIP trunking to connect to a SIP provider totally depends on the vendor.

In case your equipment requires you to get a license to use SIP trunking, you usually purchase it from the reseller that sold the equipment. For example, if you purchased a Cisco Unified Communications Manager system and want to connect it with a SIP provider, you need to purchase a Cisco Unified Border Element (CUBE) license, which is a "right-to-use" license that legally allows you to use SIP trunking to connect your Cisco equipment to a SIP trunking provider.

If your equipment doesn't require a license, you can simply find a SIP trunking provider and configure the equipment to connect to it.  No licenses are required on the provider's side; they just charge you for the simultaneous calls and usage.

Ever since telephony vendors recognized the acceptance of the Session Initiation Protocol (SIP) and their competitors introduced pure IP telephony systems like Cisco Unified Communications Manager and Avaya Aura SIP Enablement Services, they began replacing their existing traditional telephony systems with hybrid systems that offered enough VoIP/SIP functionality to remain in the market.

SIP trunking options for old telephony systems

Most new telephony systems provide some type of SIP trunking support, so if your system has been purchased recently, chances are it already supports SIP trunking. If you're stuck with an old telephony system, there are still ways to connect it to a SIP trunking provider. The legacy PBX is then configured to route calls through the POTS/ISDN interfaces directly to the IP telephony/SIP Gateway, which in turn sends the call to the configured SIP trunk provider.

SIP Trunking Explained

You've got the basics, now read the rest:
Understand the basics of SIP vs. PRI
Calculating VoIP bandwidth  for SIP trunking
Highlight reel: SIP trunking advantages
Security checklist for SIP trunking services

The legacy PBX (see diagram 1) on the left has telephones attached to it.  An IP telephony gateway is used to connect the legacy PBX to a SIP provider. The gateway interfaces with a legacy PBX via POTS or ISDN. From the other side, it connects to the company's IP network via Ethernet.

Similarly, incoming calls from SIP trunking providers are routed from the gateway to the legacy PBX via the POTS or ISDN interfaces that connect the two. A SIP provider can be either an Internet SIP provider, which uses an existing dedicated Internet connection for calls, as in diagram 1, or it can bring a dedicated line into the customer's premises (see diagram 2) to connect directly to the customer's equipment.

Legacy PBX connected to SIP provider over the Internet
Legacy PBX connected to SIP Provider via dedicated line

Depending on the capacity required for simultaneous calls to and from the SIP provider, companies need to choose the type of connection between legacy PBX and IP voice gateway.  In the above example, two Basic Rate Interface (BRI) ISDN lines connect the PBX with the IP gateway. ISDN provides up to two simultaneous calls to get a max of 2x2 = 4 simultaneous call capacity between the PBX and IP gateway.

The capacity between the IP gateway and SIP provider is limited only by the available bandwidth between them.  Remember that SIP trunking providers don't provide unlimited channels (simultaneous call sessions).  Your organization will usually be charged for the number of channels you want to use through the SIP trunk -- assuming there is enough bandwidth.

Zeroing in on the SIP trunking provider for you

Searching for a SIP provider is an easy task for one reason: There are thousands of them all over the world.  As noted previously, the two usual methods of connecting to a SIP provider is via a dedicated data line or using an organization's existing network infrastructure to connect over the Internet. Dedicated lines are preferred by companies that require guaranteed quality and minimum downtime. These types of connections are usually accompanied by a service-level agreement (SLA).

Connecting via the Internet is the fastest, cheapest and most flexible option, but it is based on best-effort delivery without any guarantees. Connecting to a SIP provider via the Internet is as easy as signing up for a SIP trunk account, selecting a local number and registering your telephony system.  The whole process can be completed within an hour or two. Getting a dedicated data line directly from the provider requires days, if not weeks, and costs a lot more money.

If connecting to a SIP trunking provider over the Internet sounds good, remember that the service is likely to experience quality issues at some point -- especially during peak traffic hours when congestion is most likely to occur.

Finally, keep in mind that not all SIP trunking providers are the same. Some have large networks with plenty of resources and the ability to handle thousands of calls per customer, while others have smaller networks with limited capacity, which can also limit the quality of your SIP calls.

Generally, if you are looking at purchasing a SIP trunk that will support only a few voice channels (two to six, for example), then Internet-based SIP providers might offer a flexible and cheap solution without a contract.

If your company's requirements include a large number of voice channels, it might be worth looking for a SIP provider that will terminate its end of the SIP trunk on equipment located on your company's premises (see diagram 2). This type of setup ensures greater stability and availability because your SIP traffic won't flow over the Internet, but rather from your network directly into the SIP provider's network.

Next: Part three of our SIP Trunking Explained series looks at bandwidth requirements to provide high-quality VoIP over a SIP trunk.

Next Steps

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