How Many SIP Trunks Do I Need?

Mo’ Calls, Mo’ SIP Trunks

In our last discussion about SIP Trunk set-up and monthly charges, we covered all of the factors that go into the monthly price of SIP trunking and how that compares to traditional phone lines.

Today we’ll look at how to calculate the number of SIP trunks that your VoIP Solutions Provider will need to install. 

FYI, A SIP Trunk (also known as a “call path”) supports one phone call to the outside world. So basically the more calls your office makes, the more SIP Trunks you need. 

How Many SIP Trunks Do I Need?

When determining how many SIP Trunks your office may need, there are several factors that can help guide your decision.

Here’s a few of main ones: 

Is Your Current Number of Call Paths Sufficient?

Do customers or employees ever complain about busy signals or not being able to get an available line for an outgoing call? If so, find out how many call paths you currently have. (If you are using traditional telephone technology, these are known as PRI circuits)

Get Usage Reports from Your Current Provider

Some traditional providers can sometimes produce reports that show usage. But because they’re not from fully digital sources, usage reports can sometimes be unreliable – often only providing only a snapshot of usage rather than quality historical information.

The 3:1 Rule for Larger Companies

For companies with 100 employees or more, a traditional ratio of phones to call paths is 3:1.

Why? Several reasons:

  • These kinds of companies have a range of heavy, medium and light users.

  • On average, even heavy users on the phone no more than three hours per day

  • Typically, not everyone is in the the office at the same time.

Bottom line, for larger companies, it’s extremely rare for everyone to be on the phone at the same time. Therefore, you can get away with a higher phone to trunk line ratio. 

Often larger companies need to use complex formulas to determine the number of call paths they need. If you’re a glutton for punishment, you can learn about them from this detailed explanation and calculator on Erlang Telecom Load Calculations.

Smaller Companies Need a Lower Ratio

But for smaller offices of less than 50 people, the 3:1 rule breaks down.

And it’s easy to see why.

The smaller the office, the greater the chance that everyone is going to be on the phone. And in fact, for very small offices you often need more SIP Trunks than you have phones. 

Use this little table as a rough guide:

# of Phones SIP Trunks Needed
2 4
5 6
10 6
25 12
50 23
100 30

This post is meant a general guide. Every office and business has their own unique requirements. 

If you have don’t want to get a telecommunications engineering degree just to figure out how much call capacity your office needs, give us a call or send us an email. One of our technicians will be glad to help you!

We offer business-class communication solutions with responsive local support

To learn more, call us today at (510) 785-2480