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How Many SIP Trunks Do I Need For My Small Business?

Wondering “How many SIP trunks do I need?” That might be because you’ve made the wise choice of slashing your phone bills by 40%. In fact, up to 80% of companies with 20,000 employees said SIP trunking is crucial to their business.

Yes, SIP trunking can save you 90% on international calls for your small business. Luckily for you, SIP trunking does not run like the traditional telephony operations, so you won’t need a bunch of lines.

One SIP trunk may be perfect, but that would be determined by the size of your business and how much communication the company handles.

A factory’s SIP trunking needs may be significantly less compared to a telecommunication company’s need. The number of trunks will determine the rate at which calls flow and the effectiveness of work in the station.

If your organization handles lots of communication then you need more trunks. You don’t want to sacrifice the quality of your communication because you’re pinching pennies! Let’s explore further considerations.

Wrong Answers to “How Many SIP Trunks Do I Need?”

When the question “How many trunks do I need?” comes to mind, for most vendors, they’ll rather stick to the old rule of thumb — one trunk for three employees. For others, they take it as duplicating the current number of trunks you’re using (in cases where it applies). Using these methods can lead to the wrong answers.

In our days, when an entrepreneur asks “How many SIP trunks do I need?” first he needs to understand the basic principles. The number of PRI/T-1’s or POTS lines determines the number of call paths you can make between your PBX and the Public Switched Telephone Network (PTSN). It is done through the Bell Central Office.

Small companies have no problem. Your company’s receptionist may have a good idea of how many calls your company makes simultaneously while carrying out business activities.

In a small business, you can set up a survey to know how many employees make calls simultaneously. This inquiry offers you the background information and provides you a straightforward process of answering the question of “How many SIP trunks do I need?”

When you’re sure how many calls happen at the same time in your office, add some room in case it goes past that. Increase the numbers you could get from your direct investigation by 20% to be on the safe side.

Number of Concurrent Calls Fall with Increase in Number of Users

As the number of employees increases in size, the percentage of people using phones would reduce. If there are two users in a work environment, both of them can make calls at the same time. Two trunks can be used.

If there are 10 people, only 50% of them can be on current calls while using five trunks.

For small companies, it’s not hard to answer the question, “How many SIP trunks will I need?” as the employees will make and take calls concurrently without hitches.

With this in perspective, it is simpler to calculate, and your vendor may be able to figure this for you. Hence, your small company presents you an advantage of having all your employees on the phone at the same time just like the traditional telephony situation.

However, the population size in relation to the number of trunks is inaccurate sometimes and should not be relied upon. The one-to-one ratio may be appropriate for some centers but not for others. Asides conversation time of the staff, other factors come to play. Such factors include ring-time, delay time, and after-call work.

Models For Determining Trunking Requirements

The question of “How many SIP trunks do I need?” is in itself a process of determining staffing needs, calculating trunk requirements, and using other models to understand what happens on the telephone trunks to the callers.

When considering staffing needs, Erlang C is the perfect model of choice. For calculating trunk requirements, Erlang B is the model to put to work.

Erlang B is a great model, but it’s limited to situations where there is no blockage. If there is a blockage, the Erlang B calculation will be inaccurate. It works based on the assumption that if a call is put through to a set of trunks and they are not available, then they’ll quit trying.

This will belittle the need for trunks and give wrong figures in a calculation.

For the limitation of the Erlang B calculator is the extended Erlang B calculator; it takes into consideration situations where a large number of callers will try calling again. It is a derivative of Erlang B and puts into perspective the number of times the caller retried.

Using an Erlang B Calculator

Here’s an easy guide to using the Erlang B calculator. The first thing is to ask or find answers to how many times could a caller get a busy signal?

For most calculations, it is always assumed that there is a one percent chance of receiving a signal that is busy.

Estimate the traffic load in Erlangs or BHT. One line session busy for one hour equals one Erlang or Busy Hour Traffic (BHT).

To determine the traffic load, you must first find your Erlang load, expressed in minutes. You must also be able to estimate the number of calls expected. You’ll want to calculate this during the busiest part of your workday.


The Erlang load (BHT) = CAR X H/60.

To break it down:

Call arrival rate (CAR) is the number of calls you operation receives during your peak hour of performance. a

The mean call length is also known as the holding (H) time. It is recorded in minutes.

To Wrap It Up

Sometimes, the call center is not capable of determining how many trunks a company needs. This duty would shift to the IT department.

The call center should, however, work with IT department in such instances, to ensure miscalculations from the presentation of wrong figures do not occur. A couple of times, staffing poses a problem in calculations but this is not the case for small companies where estimates are much easier.

If you’re interested in learning more about your SIP trunking options, then offers you a robust resource. See our blog here for more information!