SIP Trunking for Dummies: What Are My Simultaneous Call Needs?

If you’re considering switching to SIP Trunking for your business voice communications, you’ve no doubt asked yourself how many trunks you’ll need to support your business.  

One of the first steps in determining the number of SIP Trunks that you’ll need is calculating your trunk requirements. 

This is done in 3 easy steps:

1. Calculate Your Trunking Needs

Incoming Calls

Answering the question; “what is my calling volume?” can pose a challenge to many small and medium businesses that are not running call centers or monitoring their traffic with call accounting software.

When trying to size a SIP solution, many vendors just use an old, but solid “rule of thumb” of 1 trunk for every 3 employees. Unfortunately, that can lead to some wildly inaccurate and wasteful provisioning depending on whether you are a taxi dispatch or an office of engineers.

We often find that the best person to ask, if possible, is your receptionist. The next most reliable is monitor the number of outgoing calls during your busiest times. 

Outbound Calls

This is calculated by estimating how many people are originating calls (versus answering incoming calls) during a busy hour and how long they are on the phone.

Once you understand your SIP Trunking needs, how do you determine the bandwidth you will need for those calls?

2. Choose A Protocol

Because a voice call over the data network can be of varying clearness, there are protocols that will handle the extra signaling and sampling, to make sure that your company’s voice quality will be acceptable.

Use your SIP provider to help you determine the level of quality your company requires. You may not need the highest level level of bit rate or sampling to get good voice quality.

You don’t want to waste bandwidth you don’t need.

While your vendor should be able to tell you what the best protocol is for your calling needs, here is a chart, courtesy of  Gary Audin as was originally posted in that should give you a good idea of your bandwidth needs.

As you can see, there is a large difference between the protocols and the first tendency is to either be stingy with your bandwidth and hope the connection is clear enough for your company needs or to over-design for the highest clarity.

Keep in mind that most IP trunk providers will recommend a full 20% additional bandwidth to be available to cover administrative traffic, control and signaling.

Bottom Line: Always estimate on the high side.

3. Putting It All Together

Once you get a good idea of your trunking needs, it’s time to calculate how much bandwidth you will need to accommodate those SIP Trunks.

simple SIP Lines to VoIP Bandwidth calculator like the one pictured below will give you some estimates that you can discuss with your SIP provider.

 You can select the coding algorithm and the speech sample time (see chart above) and either your trunk requirements OR your available bandwidth to see how many trunks can fit over available or future bandwidth. Just remember that these calculations don’t include the suggested signaling and administrative overhead.


Finding out how many SIP Trunks you need is just the first step. Download our SIP Trunking Checklist to make sure you’ve got all the bases covered. 

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