When you ask SIP trunk providers for a quote, the first thing they ask is how many channels you want. It sounds simple. Get it wrong, though, and you either pay for capacity you never use or turn callers away with a busy signal.
This article explains what a channel is, how to count what you need, and how to size your internet connection to match.
What Is a SIP Channel?
A channel is one simultaneous call. If you have eight channels, eight calls can be live at the same moment, whether inbound or outbound. The ninth caller gets a busy tone or is diverted, depending on your setup.
It isn’t the same as a phone number or a user. You can have 50 numbers and 100 staff on only 12 channels. What matters is how many people are on calls at once.
This is a key difference from ISDN, where capacity came in fixed blocks of lines. With SIP trunk Providers you can usually adjust channels more flexibly.
Why Headcount Is the Wrong Starting Point
Many businesses assume one channel per employee. That is almost always too many.
Think about a 30-person office. At any moment, most people are in meetings, writing, or away from their desks. The number on the phone at once is usually a fraction of the total. The exact fraction depends on the type of work, not the size of the team.
A telesales floor is the opposite. If 15 agents dial constantly, you need close to 15 channels plus headroom.
How to Work Out Your Peak Concurrent Calls
You need to find your busiest moment, not your average. Here is a practical method.
- Pull call records. Your current provider or PBX can usually export call logs with start and end times for the last few months.
- Find the busiest periods. Look for the highest number of overlapping calls. Pay attention to Monday mornings, month-end, deadlines and seasonal peaks.
- Include all call types. Count inbound, outbound, transferred calls and calls on hold. A held call still uses a channel.
- Allow for overflow. Add headroom for growth and unexpected spikes. A common planning approach is to add a margin on top of your observed peak, and your provider can advise on a sensible figure for your pattern.
If you don’t have logs, ask each team lead how many people are typically on calls together at the busiest time, then add a safety margin.
Things That Quietly Use Extra Channels
A few features can consume more channels than people expect.
Call forwarding to mobiles. When a call is forwarded out, it often uses two channels: one in, one out.
Call transfers to external numbers. The same applies.
Auto-attendants and queues. Callers waiting in a queue are still on a call, so a busy queue can fill your trunk quickly.
Call recording and conferencing. Depending on setup, these can add legs to calls.
Ask your provider how forwarding and queues are counted so you aren’t surprised.
Sizing Your Internet Connection
Each channel needs bandwidth in both directions. The amount depends on the audio codec. Uncompressed G.711 audio typically needs roughly 80 to 100 kbps each way once overhead is included, and compressed codecs use less.
As a rough illustration, 20 simultaneous calls on G.711 would call for around 2 Mbps of dedicated upload capacity and the same for download. That is only a ballpark, so ask your provider for exact figures for the codec they use.
The important point is that upload capacity is often the limit on business broadband, and voice shouldn’t compete with large file uploads or backups. Where possible, prioritise voice traffic on your router or use a separate connection for calls.
Examples for UK Businesses
A 12-person estate agency finds that its busiest hour, Monday at 9:30am, has five overlapping calls. It chooses eight channels, leaving headroom for forwarded calls and growth.
A 25-seat telesales team sees up to 20 agents on outbound calls at once. It buys 24 channels and discusses burst capacity with its provider for campaign weeks.
A three-site training company has 60 staff but rarely more than 10 concurrent calls across sites. It shares a pool of channels across locations rather than buying separate capacity for each.
Where Wavetel Business Fits In
Some suppliers, including Wavetel Business, let you adjust channel counts as your needs change instead of locking you into a fixed block. Whoever you choose, ask how quickly channels can be added and whether pooled capacity across sites is available.
Frequently Asked Questions
How many channels do I need per employee?
There’s no fixed ratio. Base it on your peak simultaneous calls, then add headroom. Office teams often need far fewer channels than staff, while call centres need close to one per agent.
What happens if I run out of channels?
Additional callers hear a busy tone, or calls are diverted if you’ve set up overflow routing to another number or voicemail.
Can I change the number of channels later?
Usually yes. Many providers can add or remove channels with short notice, though terms vary, so check your contract.
Does a SIP channel need its own phone number?
No. Numbers and channels are separate. One set of channels can serve many numbers.
Conclusion
Channels measure simultaneous calls, not staff or numbers. Find your true peak from call logs, remember forwarded and queued calls, add sensible headroom, and check your upload bandwidth.
Your practical next step is to export a month of call records, find the busiest hour, and use that figure as the starting point when you ask providers for quotes.