---
title: "Some calls fail when you're busy"
url: "https://enfonica.com/help/call-troubleshooting/calls-fail-when-busy"
summary: "If an outgoing call fails only while other calls are in progress, your SIP domain has used all its channels, which set how many outgoing calls it allows at once. Raise the number of channels on your SIP domain in the Enfonica Console. The change applies within about a minute."
type: troubleshooting
audience: everyone
products: ["Cloud SIP", "Teams Direct Routing"]
works_with: ["Any SIP PBX or SBC", "Microsoft Teams Direct Routing"]
who_can_do_this: "A project Owner or Admin, or whoever manages your PBX"
---

# Some calls fail when you're busy

**Short answer:** If an outgoing call fails only while other calls are in progress, your SIP domain has used all its channels, which set how many outgoing calls it allows at once. Raise the number of channels on your SIP domain in the Enfonica Console. The change applies within about a minute.

**Applies to:** Cloud SIP, Teams Direct Routing  
**Works with:** Any SIP PBX or SBC, Microsoft Teams Direct Routing  
**Who can do this:** A project Owner or Admin, or whoever manages your PBX

First check the [Enfonica status page](https://enfonica.status.io/) for a known incident.

## A second outgoing call fails while another is in progress

What you see: `SIP/2.0 503 Channel Limit Reached`

If calls work one at a time but the next call fails while others are in progress, your SIP domain has used all of its channels. A channel is one outgoing call in progress. Your plan sets how many channels your SIP domain can use at once. When every channel is in use, Cloud SIP refuses the next outgoing call with this response. Incoming calls keep working, because they don't use channels.

### Your plan has fewer channels than the calls you make at once

This is the most common cause. Starter, Business, and Per user plans include a set number of channels or users. A busy moment or a new team member can use the last of them.

**To check**, look at the failed call and the other calls at that moment. This is the cause if both of these are true:

- Your PBX's call log or SIP trace shows `503 Channel Limit Reached` for the failed call.
- **Cloud Voice > Calls** in the Enfonica Console shows other outgoing calls in progress at that moment.

The refused call itself has no record in the Console, because Cloud SIP turns it away before it starts.

To see how many channels you have, go to **Cloud SIP**, open your SIP domain, and look at **Bundle plan**. It says, for example, "You can use up to 2 outgoing channels."

**To fix it**, add channels to your SIP domain:

1. In the Enfonica Console, go to **Cloud SIP**, and open your SIP domain.
2. Next to **Bundle plan**, select **Edit**.
3. Select a plan.
4. Move the slider to the number of channels (or users) you need.
5. Select **Save**. The new limit applies within about a minute.

More channels change what you pay each month. For current prices, see our [pricing page](https://enfonica.com/pricing).

### Your Teams Direct Routing SIP domain still has its starting limit of 1

Every new Teams Direct Routing SIP domain starts with a channel limit of 1. That allows one outgoing call at a time across all your Teams users.

**To check**, look at when Teams calls fail. This is the cause if outgoing calls from Teams fail when two people call out at the same time, while incoming calls ring normally.

**To fix it**, go to **Cloud SIP** in the Enfonica Console, and open your Teams Direct Routing SIP domain. If it shows **Bundle plan**, change the number of channels there, as in the previous steps. Otherwise, follow these steps:

1. In the **Teams Direct Routing** section, set **Channel limit** to the number of simultaneous outgoing calls your users make, from 1 to 100.
2. Select **Save Changes**.
3. To confirm, select **Increase**.

## Calls fail at high volume on pay-as-you-go

What you see: `SIP/2.0 503 Channel Limit Reached`

A standard SIP domain on pay-as-you-go has no channel limit of its own, so a busy office won't run out of channels. Contact centers and dialers can reach the limits on the account itself.

### Your calls reached your account's limits

Each account starts with a limit of 100 concurrent calls and 10 outgoing call attempts per second. There's no limit on how fast incoming calls can arrive.

**To check**, look at when the failures start. This is the cause if they start when a campaign or dialer ramps up, with close to 100 calls in progress. A dialer that places calls faster than 10 a second can also reach the limit on call attempts.

**To fix it**, ask us to raise these limits. Contact your account manager or our support team with the number of concurrent calls and call attempts per second you expect at your busiest. Until the new limits are in place, lower the dialer's pacing.

## Calls fail with a 503 that doesn't say Channel Limit Reached

What you see: `SIP/2.0 503 Service Unavailable`

A 503 with any other text isn't about channels, even when it happens at a busy time. The words after the code tell the causes apart.

### The 503 has another cause

**To check**, read the full response. This is the cause if it doesn't include `Channel Limit Reached`, or if calls to one destination fail while other calls go through at the same moment.

**To fix it**, see [SIP 503 Service Unavailable](https://enfonica.com/help/sip-errors/503-service-unavailable). It covers each of the other causes, such as an account out of credit or a fault in the network beyond Enfonica.

## How do channels work?

A channel is one outgoing call, counted from the moment your PBX starts the call until it ends. Calls that are still ringing count too. Incoming calls never use a channel, so a full set of channels never stops people from calling you.

Channels belong to the SIP domain, so everyone who calls out through it shares them. Your plan sets how many there are.

| Plan | Outgoing calls at once |
| --- | --- |
| Pay-as-you-go (standard SIP domain) | No limit on the SIP domain. Your account's limits apply. |
| Starter | 1 to 4 channels |
| Business | 5 channels or more |
| Per user | Up to the number of users on the plan |
| Teams Direct Routing SIP domain | The channel limit you set, starting at 1 |

To choose a number of channels, count the most people who make outgoing calls at the same time on your busiest day. For plans and prices, see our [pricing page](https://enfonica.com/pricing).

## Questions

### Do incoming calls use channels?

No. Channel limits apply to outgoing calls only, on every plan. Incoming calls are never refused because your channels are in use.

### How soon does a change to my channels apply?

Within about a minute of saving it.

### What happens if I reduce my channels?

Channels are billed in advance, so you can keep using the higher number until the current billing period ends. While this applies, the Console shows a note under **Bundle plan**.

### Which plan suits a contact center or dialer?

Pay-as-you-go. Bundle plans can't be used for call center, telemarketing, or outbound sales calling. On pay-as-you-go, your account's limits apply. Tell us the volume you expect, and we can raise your account's limits to suit it.

## What to send us

Include these and we can find the call straight away.

- The number that made the call and the number it called, in full international format (for example +61255501234).
- The date and time of the call, with your timezone.
- Whether the call was inbound or outbound.
- The call ID, if the Console shows the call.
- What happened: an error message, silence, a busy tone or a dropped call.

[Contact support](https://enfonica.com/contact-support?article=%2Fhelp%2Fcall-troubleshooting%2Fcalls-fail-when-busy&topic=call-fault)

## Related

- [SIP 503 Service Unavailable](https://enfonica.com/help/sip-errors/503-service-unavailable.md)
- [Outbound calls aren't going through](https://enfonica.com/help/call-troubleshooting/outbound-calls-fail.md)
- [Find a call and read its SIP trace](https://enfonica.com/help/call-troubleshooting/find-call-sip-trace.md)
- [Connect Microsoft Teams Direct Routing to Enfonica](https://enfonica.com/help/teams-direct-routing/connect-teams-direct-routing.md)
- [Cloud SIP limits (docs)](https://enfonica.com/docs/cloud-sip/limits/)
- [Pricing](https://enfonica.com/pricing)

---

Web version: https://enfonica.com/help/call-troubleshooting/calls-fail-when-busy