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How to Estimate the Right SIP Trunk Capacity for Your Business

Author : Andrew Johns | Published On : 22 Aug 2026

When you choose the right SIP trunk capacity, you pay for the right capacity you need to handle calls—not paying for extra capacity that will not be needed. The key is to estimate how many calls your business can handle simultaneously, especially during busy periods.

A SIP trunk channel supports one simultaneous call. So, if 10 staff members make calls at the same time, you need capacity for at least 10 concurrent calls.

However, the number of employees you have does not tell you how many SIP channels you need.  What matters is how many people are likely to be on calls simultaneously.

What Is SIP Trunk Capacity?

SIP trunk capacity is the number of simultaneous calls your SIP trunk can handle.

For example:

  • 4 channels = up to 4 simultaneous calls

  • 8 channels = up to 8 simultaneous calls, and so on. 

If all your employees are using all channels, there won't be a free one for another worker to use. Another call may not be able to connect until a channel becomes available.

This is why you should size your SIP trunk based on peak call demand. This is the total number of calls your business can make without exceeding on busy business days, rather than the total number of employees.

Central Connect offers 4-, 8- and 16-channel SIP trunk plans, as well as custom capacity for larger or higher-volume businesses.

How Many SIP Trunk Channels Does Your Business Need?

The simplest way to estimate your requirement is:

SIP trunk capacity = maximum number of simultaneous calls you expect

For example, if your business has 20 employees. During its busiest period, it expects:

  • 3 sales staff on calls

  • 2 customer service staff on calls

  • 1 manager on a call

  • 1 receptionist handling a call

That is a total of 7 simultaneous calls.

So you need an 8-channel SIP trunk with sufficient capacity to handle the total number of calls during the expected peak period. Plus one additional channel available. The key point is that 20 employees does not mean you need 20 channels. 

Step 1: Find Your Peak Call Volume

Start by looking at when your business receives and makes the most calls. Estimate how many calls happen during that period. 

For example, a business might handle 100 calls a day, but at most 20 occur at the same time. What matters for SIP capacity is how many calls overlap.

If you already have a phone system, check its call records for the highest number of simultaneous calls. Then, if you are setting up a new phone system, know how many employees will be on external calls during the busiest period.

Step 2: Consider the Type of Business

Different businesses have different phone usage regarding calls. Some need less calls, others need more calls. 

For instance, a small office might need to make only a few calls. While a sales team, customer support department or contact centre may need many more.

Let’s assume: 

General office: most employees hardly make calls. They email, chat and that’s it.

Sales team: Staff may spend much of the day making outbound calls discussing with clients. 

Customer service team: Several employees may handle customer calls simultaneously, particularly during busy periods.

Call centre: Almost every agent may be on a call during peak periods. 

So your SIP trunk capacity should therefore be based on actual employee activity. Which are calls they make simultaneously. Rather than a general usage. 

This is why rules such as "one channel for every three employees" should only be treated as a starting point. Actual call behaviour is more useful.

Step 3: Add Some Spare Capacity

You should size your SIP trunk exactly to your current peak simultaneous calls. However, leave space for extra calls. 

Suppose during your busiest period you make 7 simultaneous calls. An 8-channel plan gives you a small amount of additional capacity for an unexpected spike in calls.

For larger businesses, you may want a bigger buffer to account for:

  • Unexpected call spikes

  • New employees 

  • Marketing campaigns 

  • Seasonal demand

  • Business growth

All of these can lead to more calls than your peak. How much extra capacity you need depends on your call volume. 

For example, if a large business typically handles 50 calls at once but can reach 70 during call spikes, it may need capacity for at least 70 calls.

Step 4: Check Your Internet Bandwidth

You should have enough SIP channels, but your network needs to handle them.

Voice calls use bandwidth. The amount you need for calls depends partly on the codec your phone system uses.

For example, G.711 uses around 64 kbps for the voice stream. Before other network data is added. The call needs slightly more bandwidth because additional network data is also sent with the voice.

How you plan the right bandwidth, use this simple calculation:

Required voice bandwidth = simultaneous calls × bandwidth per call

For example, if you expect 10 simultaneous calls and your setup uses about 80 kbps per call for voice traffic:

10 × 80 kbps = 800 kbps

That is only the estimated voice traffic. Capacity for normal business activities such as cloud applications, video meetings, file transfers, and web browsing is not accounted for. 

So, do not choose an internet connection based on the bandwidth required for your SIP calls. Plus other internet activities. 

Step 5: Check Both Inbound and Outbound Calls

When doing capacity planning consider incoming and outgoing calls to your business.

For example, a business may have five customer service employees answering inbound calls and five sales employees making outbound calls. That is 10 calls could happen at once. The business needs capacity for 10 concurrent calls, not five.

This is particularly important for businesses using their phone system for sales, customer service and reception activities.

Step 6: Think About Growth

Your current call volume may not be your future call volume. Your business might grow having more customers and need more staffing to tend to them. It will increase your peak number of simultaneous calls.

For example, if your business currently makes six concurrent calls but plans to add a 10-person sales team. Then your future requirement could be much higher.

With SIP trunking it is easy to add capacity based on changing call demand. Central Connect, for example, offers channel bundles that can be scaled from smaller plans to larger or custom configurations.

The Bottom Line

The right SIP trunk capacity depends on how many calls your business needs to handle simultaneously, not simply on how many employees you have.

The question is: during your busy periods, how many calls do you make concurrently? Then add reasonable room for unexpected demand and future growth. Then you can estimate how much SIP trunk capacity you need. 

Also ensure your internet connection can support the required voice traffic alongside your other business applications.

For a small business with no more than 5 staff, a 4- or 8-channel plan may be enough. A business with several departments may need 16 or more channels. A business with very high call volumes may need a custom SIP trunk plan. One offering more channels than the standard plans offer.

Central Connect provides 4-, 8-, 16-channel and custom SIP trunk options to accommodate different business needs.

The goal is simple: enough capacity to handle your busiest periods without paying for channels your business does not need