Synthflow Call Hangup Reasons

End Call Reason

Description

Next Steps

Possible Call States At Hangup

Insufficient Minutes

This issue occurs when you've used up all the minutes included in your current plan.

What You Can Do

If you're on the Starter plan, you can upgrade to a plan with more minutes that better fits your needs:

Go to:
Settings → Plan and Billing → Upgrade

Plans starting from the Pro tier also let you enable Pay-As-You-Go, which automatically recharges your balance to prevent service interruptions.

To turn this on:

Go to:
Settings → Usage → Auto Recharge → Toggle "Enable"

Once enabled, make sure to set:

• Trigger Threshold – the balance at which auto-recharge should activate (e.g., when the balance drops below 10 minutes)

• Recharge Amount – how much credit to add each time (e.g., add $50)

This ensures your assistant stays active without any downtime.

Failed

429: Rate limit exceeded. Your rate limit per minute is XX. Contact Sales to increase the rate limit.

This error occurs when the number of calls you're attempting to place exceeds the limit allowed by your current plan.

You can either upgrade to a plan that supports a higher number of concurrent calls or implement call queuing on your end to prevent failures.

If you require additional concurrency capacity, we recommend reaching out to our Sales team to explore custom solutions tailored to your needs.

Failed

608: Rejected

This error typically occurs when the end-user has enabled a spam-blocking feature, leading the operator to reject the call.

This error typically occurs when the end-user has enabled a spam-blocking feature, causing the call to be rejected by the carrier or operator.

To reduce the likelihood of this happening:

• Use a verified and recognizable caller ID.

• Maintain a positive call reputation by following consistent, non-intrusive calling patterns.

• Additionally, consider submitting your phone numbers to the FCC or Hiya registries to request whitelisting across carrier networks.

Failed

404: Not Found

The destination number may be invalid or out of service.

Please verify the accuracy of the destination number.

Failed

487: Request Terminated

The destination number disconnected the call without answering it.

Retry again after some time

Failed

400: It was not possible to start the call because of a Twilio error: Unable to create a record: The caller ID phone number provided, +123456789, is not yet verified for your account.

On a Twilio trial account, every outgoing number (both the “To” destination and the “From” caller ID) must be added and verified in your console before you can use it. This error indicates that the phone number is not verified

Verify the Phone number that you are dialing or the caller ID that you are using on the Twilio platform.

Failed

400: It was not possible to start the call because of a Twilio error: Unable to create a record: Account not allowed to call +123456789

This error occurs when you are unable to dial a particular number because of the following reasons:

1. It could be blocked by DND.

2. There could be Geo-permission restrictions

3. You are on a Trial account with Twilio, and the number you are trying to call is not verified.

1. Geo-permissions not enabled

Twilio blocks international calls by default to prevent abuse.

If +123456789 is in a country you haven’t explicitly allowed, the call will be blocked.

Fix:

Go to the Twilio Voice Geographic Permissions settings.

Enable outbound calling for the specific country or region.

2. Trial Account Restrictions

If you’re using a Twilio trial account, you can only call phone numbers that have been verified manually in your Twilio Console.

Fix:

Go to your Twilio Console → Verified Caller IDs, and add the number you're trying to call.

3. Regulatory / Compliance Blocks

Some countries require local compliance (e.g., caller ID registration or regulatory documents).

Fix:

Contact Twilio Support or check country-specific requirements in Twilio's regulatory guidelines.

Failed

400: It was not possible to start the call because of Twilio error: Invalid 'to' phone

This error occurs since the number you are trying to dial is incorrect or incorrectly formatted.

Note - We support dialing in E.164 format only.

Ensure the number that you are dialing is correct.

Failed

400: Error creating the agent: Too many requests to GHL API

This error occurs when you are making multiple API requests to your GHL Custom actions.

• Ensure you have not linked the same custom actions to multiple assistants calling the API multiple times.

• Work with the GHL Team to review these API errors to either increase the API limit or share inputs on the error if it's unclear.

Failed

400: Error creating the agent: Illegal header value b'Bearer

This error occurs when the Real Time Booking action is linked to the assistant, but the relevant account or the subaccount is not linked to the custom action.

If you are using GHL for Real-Time Booking, ensure the right subaccount is connected to the custom action, which allows the assistant to execute this action without errors.

You can verify the connection by navigating to

Integrations -> Third parties -> Go High Level or Cal.com is connected.

Disconnect and connect again before retrying.

Failed

403: Forbidden

This error can occur if you are dialing a destination that the custom Twilio account doesnt support.

Follow the instructions in this article to adjust the Geo-permissions to enable dialing towards a destination of your choice.

Failed

The Voice ID is not accessible to this API key. Please add this voice to your ElevenLabs voice library.

This error occurs when you have connected your custom ElevenLabs account and are using a Voice that is not available in your ElevenLabs account.

Ensure the Voice that is available in your ElevenLabs account is linked to the assistant.

Failed

Background noise environment is required when the background noise volume is greater than 0. Go to the agent settings and select a background noise environment or set the volume to 0

This error occurs when the Background noise is selected but the volume is set to 0.

Adjust the slider to a non-zero value.

Navigate to Assistant -> Configure -> Call configuration -> Background noise -> Change the value to a non-zero value.

Failed

Forbidden

This error can occur if you are dialing a destination that we do not support.

Connect your own Twilio account to be able to make calls to a destination that we do not support.

Failed

Something went wrong

This error can occur for a multitude of reasons. Reach out to our Support Team for help.

Reach out to the Support team with:

1. Workspace ID

2. Assistant ID

3. Call ID

Failed

Call Failed / Unknown error

This error needs to be investigated as the underlying error could be unique.

Reach out to the Support team with:

1. Workspace ID

2. Assistant ID

3. Call ID

Failed

You have exceeded the token rate limit of your current OpenAI pricing

This error occurs when you have connected your own OpenAI account, and this account has hit the maximum number of tokens (input + output) allowed per minute or day, according to the rate limits of your OpenAI pricing tier.

1. Check your OpenAI plan's rate limits and upgrade your plan (if needed).

2. Disconnect your OpenAI account to ensure the calls do not continue to fail.

Failed

LLM token rate limit exceeded : Azure OpenAI token‐per‐minute limit exceeded; retry after X seconds or upgrade tier.

This occurs when your Azure OpenAI has exceeded the token-per-minute (TPM) limit allowed by your current Azure OpenAI resource tier.

Upgrade your tier / request a quota increase

Failed

Human Goodbye

This indicates the call is completed successfully, and the person disconnected the call.

Completed

Voicemail

This indicates the call was answered by voicemail.

Completed