How to Calculate How Many SIP Trunks Do I Need?

If your business is considering making the move to SIP Trunking to cut call costs, then you’re making a sound choice. It can be difficult and confusing however for executives to fully understand what is SIP Trunking, the technology underlying and what is needed before deployment. All business changes require planning, that’s a prerequisite that ensures that you gain maximum ROI and the best product that’s suited to the needs of the enterprise.

Related: What is SIP Trunking?

How Many SIP Trunks Do I Need?

First, you must make a choice SIP Trunking vs ISDN. SIP Trunking, unlike traditional telephony solutions, don’t require you to buy a bunch of lines that you may not need however, you can purchase as little as one SIP Trunk, depending on the size of your business and the nature of your communications.

Related: SIP Trunking vs ISDN

If you find that customers and employees often bemoan the fact that they get an engaged signal, or if there’s no outgoing lines available to use, then this is bound to be losing your business both money and customers. If this is the case, the first step in discovering your SIP Trunking needs will lie in how many call paths you currently have on your traditional PRI circuits. 

Usage Reports

With this in mind, your first step will be to contact your traditional telephony provider and ask if they can provide you with a usage report. You should bear in mind that as these often are based on analogue use — rather than digital — these can be somewhat unreliable.

For enterprises with more than 100 employees, the usual manner of determining call path/phone ratio is often the 3:1 model. This is because enterprises of these sizes may have a range of telephony users — some of which will barely use the phones at all, whilst others will use on a heavy basis and others still somewhere in between.

In other words, it’s not likely that a) every employee will be in the office at the same time. Some will be on leave, others off sick, some out in the field and so on and b) those that are in the office will all be on the phone at the same time (unless it’s a call center with a high ratio of call agents).

Amount of SIP Ttunks For Large vs Smaller Enterprises

It will then depend on the nature of the business as to how many SIP Trunks you require. For larger companies, it’s often the case that you can deploy a higher phone to Trunk ratio than a smaller company.

You can and should work with your chosen SIP Trunking vendor to calculate the amount of Trunks that you will need, or you can use a detailed calculator such as the one found here. Be aware that the calculator in itself can be confusing however and you may need some IT/telecoms expert help in using it. Also, don't forget to ask a vendor about SIP trunking benefits for your business. 

Related: SIP Trunking Benefits

The calculator is known as the Erlang B traffic model and is used for “dimensioning trunk routes.”

It’s based on the assumptions that:

1. There are an infinite number of sources
2. Incoming calls arrive at random
3. Calls are served in order of arrival
4. Blocked calls are lost
5. Holding times are exponentially distributed

When calculating SIP connections, you should also consider:

• Overestimating wastes revenue, needs more bandwidth and doesn’t necessarily improve call performance.
• Overdesign results in overspends on SIP licensing
• Underestimating can result in blocked and dropped calls, which can mean that your business frustrates and therefore loses customers
• Underestimating bandwidth requirements can affect call quality

Bandwidth Requirements

When it comes to determining the amount of bandwidth that you’re going to need, many factors need to be considered. It can be difficult to determine the bandwidth needed for a VoIP call session due to these factors, which include:

• Compression technology
• Packet overhead
• Network protocols

Due to this, it’s recommended that you work closely with your SIP Trunking provider to determine your exact needs. A good vendor will be able to provide you with the means for calculating the necessary bandwidth.

Amount of SIP Trunks For Smaller Businesses

For SIP Providers, it’s not as difficult to calculate how many SIP Trunks will be needed as it’s more likely that more employees will be making or taking calls at the same time. With this in mind, it’s much simpler to calculate and it’s likely that your vendor will be able to calculate this for you.

Shop around though and make sure — whatever the size of your business — that your chosen vendor knows how to calculate SIP Trunks and how many you will need. Larger enterprises will need to plan thoroughly if they are to achieve maximum ROI and all companies should look for vendors with certifications and partnerships with large companies such as Cisco, Avaya, Elastix and more.