Session Initiation Protocol (SIP) trunking has become an increasingly popular way for businesses to handle their voice communications over IP networks. With SIP trunking, companies can reduce costs by consolidating voice and data traffic onto a single network.
One question that often comes up with SIP trunking is how many concurrent calls a SIP trunk can handle. There are several factors that determine the maximum number of simultaneous calls that can be placed over a SIP trunk.
SIP Trunk Capacity
The capacity of a SIP trunk refers to the maximum number of concurrent calls it can handle at one time. This depends on a few key factors:
- Bandwidth – The amount of bandwidth available to the SIP trunk determines how much voice traffic it can carry. More bandwidth allows for more simultaneous calls.
- Codec – The codec used to encode the voice signals impacts bandwidth utilization. More efficient codecs like G.729 allow more calls per unit of bandwidth.
- Trunk size – Providers offer SIP trunks in different sizes, with larger trunks able to handle more concurrent calls.
So in general, the more bandwidth allocated to the SIP trunk and the more efficiently voice traffic is encoded, the more simultaneous calls the trunk can handle.
Common SIP Trunk Sizes
SIP trunks are often available in preset sizes from providers. Some common sizes include:
- 10-channel – Supports up to 10 concurrent calls
- 24-channel – Supports up to 24 concurrent calls
- 50-channel – Supports up to 50 concurrent calls
- 100-channel – Supports up to 100 concurrent calls
The number of channels indicates the maximum number of simultaneous calls supported. So a 10-channel SIP trunk could have 10 active calls at one time.
It’s important to note that these sizes are not definitive standards. Different providers may offer SIP trunks in different capacity increments. The bandwidth allocated per channel also varies based on the codec used.
Bandwidth Considerations
The bandwidth allocated to each channel impacts the voice quality and number of calls supported. Here are some common bandwidth allocations per channel:
- G.711 codec – 64kbps per channel
- G.729 codec – 8kbps per channel
- G.722 codec – 64kbps per channel
So with G.711, each call consumes 64kbps. That means a 10-channel SIP trunk with G.711 would require 640kbps of bandwidth. The same size trunk with G.729 would only need 80kbps.
In general, budgeting for at least 90-100kbps per call is recommended. This allows room for overhead and some flexibility in the codec used.
Estimating Required Trunk Size
To determine what sized SIP trunk you need, you’ll need to estimate your number of simultaneous calls. Here are some tips for estimating this:
- Look at historical peak usage – How many concurrent calls did you have at peak times in the past?
- Consider future growth – Add some extra capacity for expected business growth.
- Factor in call duration – Short calls free up capacity faster than long calls.
- Consider usage patterns – Are calls batched at certain times or spread evenly?
It’s better to overestimate trunk size rather than underestimate. If you underestimate, callers may get busy signals during peak times. Oversizing the trunk costs more but provides room to grow.
Using Multiple SIP Trunks
Rather than purchasing a single large SIP trunk, businesses can use multiple smaller trunks. For example, you could use five 24-channel SIP trunks instead of a single 100-channel trunk.
There are a few benefits to this approach:
- Cost savings – Smaller trunks may have lower flat monthly fees
- Flexibility – Trunks can be added or removed as needs change
- Redundancy – If one trunk fails, others provide backup
When using multiple SIP trunks, a device called a session border controller (SBC) is needed to manage the trunks and route calls between them. This adds some complexity, but provides more flexibility.
SIP Trunking Recommendations
Based on best practices, here are some general recommendations for SIP trunk capacity:
- Size for peak usage – Look at max concurrent calls during busy hours
- Allow 20-25% extra capacity – Provides overhead and growth room
- Use multiple trunks – Adds redundancy and allows easily adding capacity
- Get at least 24 channels – Provides minimum redundancy with multiple trunks
- Right-size channels – Don’t pay for large blocks if you need less
Monitoring call volume and trunk utilization over time allows you to add capacity as needed. With cloud-based systems, trunks can often be resized dynamically.
Factors That Increase Capacity
If your SIP trunks are hitting maximum capacity during peak times, there are a few ways to increase the number of simultaneous calls supported:
- Increase bandwidth – Allows more concurrent calls
- Use more efficient codec – Get more calls per kbps of bandwidth
- Add more trunks – Scale by adding trunks as needed
- Resize trunks – Upgrade to larger trunk sizes
Upgrading internet bandwidth and implementing quality of service (QoS) also helps improve voice call capacity and quality.
Hybrid Trunking
Some organizations utilize a hybrid trunking approach combining SIP and legacy analog or digital PRI trunks. This provides flexibility to handle overflow traffic.
For example, you may have:
- 50-channel SIP trunk – Handles regular traffic
- 10-channel PRI trunk – Takes overflow calls during peaks
By routing calls to the PRI trunk when SIP reaches capacity, this prevents busy signals during spikes. PRI provides reliable overflow capacity as needed.
On-Premises vs. Hosted SIP Trunking
SIP trunking can be deployed either as an on-premises solution or hosted service. With a hosted service, the SIP trunk capacity is provided by the cloud provider. This makes it easier to scale capacity up or down.
On-premises SIP trunks are sized based on the capabilities of your phone system and session border controller (SBC). Scaling requires upgrading hardware.
Cloud-based phone systems make it easier to resize SIP trunks in real-time through the management portal. However, total capacity is still limited by bandwidth and costs are based on usage.
SIP Call Admission Control
To prevent oversubscription of SIP trunks, call admission control (CAC) can be implemented in VoIP systems and SBCs. CAC limits use of trunk capacity by:
- Counting active calls
- Rejecting new calls over preset limits
- Playing busy signal to new callers
This prevents trunk overloads and call quality issues during traffic spikes. Fine-grained CAC settings allow limiting specific phone extensions as well.
Emergency Call Considerations
One important factor with SIP trunk capacity is that it must be sufficient to handle emergencies. Even at maximum utilization, emergency calls must be able to get through.
To ensure emergency call access:
- Reserve channels specifically for 911/emergency calls
- Implement emergency call preemption on SBC/PBX
- Limit utilization to ~90% of total capacity
Following these practices prevents emergency call blocking even during traffic spikes on SIP trunks.
SIP Trunk Providers
Leading SIP trunk providers include:
| Provider | Trunk Sizes |
|---|---|
| RingCentral | 25, 50, 100, 200 channels |
| Nextiva | 10, 25, 50, 100 channels |
| 8×8 | 10, 20, 30, 40, 50 channels |
| VoIP.ms | 1-50 channels |
| Flowroute | 1-1,000+ channels |
The number of channels and bandwidth allotted per channel varies between providers. Companies like Flowroute offer very flexible sizing and rapid provisioning of new numbers and trunks.
SIP Trunk Pricing
SIP trunk pricing often involves:
- Base monthly trunk fees – Based on number of channels
- Per-minute charges for calls
- Additional fees for numbers/DIDs
So cost depends on both capacity and usage. Overage charges may apply if trunk capacity is exceeded.
Hosted PBX/UCaaS providers bundle trunks with service plans. This provides predictable monthly pricing for bundled minutes and seats.
Best Practices for SIP Trunks
To optimize SIP trunk capacity and call quality, follow these best practices:
- Monitor usage and trunk metrics
- Add capacity before trunks reach 80% utilization
- Implement call admission control policies
- Separate voice and data traffic
- Prioritize voice traffic (QoS)
- Use G.729 or other efficient codecs
- Route overflow traffic to PRI trunks if needed
Proactively managing trunk capacity ensures quality of service even during peak load. Right-sizing trunks, upgrading bandwidth, and implementing QoS combines to deliver excellent voice traffic performance.
Conclusion
The number of concurrent calls a SIP trunk can support depends on bandwidth, codec, and trunk size. While providers offer trunks in common sizes like 24 or 50 channels, businesses can use multiple trunks for scalability and redundancy.
Monitoring usage over time and adding capacity during growth periods prevents service quality issues. With proactive management and overflow capacity as needed, SIP trunks can cost-effectively handle current and future voice traffic.
