I have a BYO SIP trunk connection that was working fine, but today it suddenly stopped working. Whenever a call is transferred, it gets rejected and the call drops. I also tried to reconnect the SIP credentials, but the issue remains. Here’s a log excerpt showing the error: Unable to create channel of type 'SIP' (cause 20 - Subscriber absent) Everyone is busy/congested at this time (1:0/0/1) exited non-zero on 'SIP/PS_SISTEMAS_DIGITAL-00032cac' Troubleshooting already performed: Reconnected SIP credentials Local dial test Checked AGI context and variables Impact: All transferred calls through the SIP trunk are being rejected, impacting the operation. Request: Please provide urgent support to identify and resolve this issue. https://cdn.discordapp.com/attachments/1412794645750218812/1412794646010007613/image.png?ex=68b9970c&is=68b8458c&hm=e198209d419b3aee9b91021fa19eb72c169f18dd83bcc967fa978cd3c9a515a8&

Suggestion: Check if SIP REFER (call transfer) is enabled on your SIP provider, as this is required for successful call transfers. Your transfer configuration should look like this:

json
{
  "type": "transferCall",
  "destinations": [
    {
      "type": "sip",
      "sipUri": "sip:15557654321@sip.your-provider.com"
    }
  ]
}

Enable SIP REFER in your provider's settings if it's not already enabled, then test the transfer again. [See Vapi SIP trunk documentation for details.](https://docs.vapi.ai/advanced/sip/sip-trunk#sip-refer-call-transfer) Source: - [SIP Trunking Vapi Documentation](https://docs.vapi.ai/advanced/sip/sip-trunk#sip-refer-call-transfer)

I’d like to clarify that SIP REFER is already configured the same way, and everything was working normally before. The issue appeared suddenly, without any changes on our side. Our transfer configuration hasn’t been modified, but now every transferred call is being rejected with the log below: Unable to create channel of type 'SIP' (cause 20 - Subscriber absent) Everyone is busy/congested at this time (1:0/0/1) exited non-zero on 'SIP/PS_SISTEMAS_DIGITAL-00032cac' Since it was previously working with the current configuration, it seems the problem is on the provider’s side or some change in the SIP trunk handling. Can you please check if there were any updates or changes in your SIP trunk infrastructure that could be causing these rejected transfers?

Hello, We are truly sorry for the inconvenience. Can you try these troubleshooting steps for your SIP trunk issue: 1. First, verify network connectivity. Ensure your system can reach Vapi's SIP signaling IPs: - 44.229.228.186/32 - 44.238.177.138/32[1](https://docs.vapi.ai/advanced/sip/sip-trunk) 2. For SIP REFER (call transfer) issues: - Check if your transfer URI is properly formatted:

sip:transfer-number@your-telecom-provider-domain.com

- Ensure the number is in E.164 format with proper '+' prefix - Verify SIP REFER is enabled on your provider's side 3. Check your SIP trunk configuration:

{
  "type": "transferCall",
  "destinations": [
    {
      "type": "sip",
      "sipUri": "sip:+[number]@[your-sip-domain]"
    }
  ]
}

4. The error "Subscriber absent (cause 20)" typically indicates: - The destination number is unreachable - Authentication issues with the SIP provider - Incorrect routing on the provider side Recommended actions: 1. Verify your SIP provider's status and service availability 2. Check if your SIP authentication credentials are still valid 3. Test a direct call (non-transfer) to verify basic connectivity 4. Ensure your provider hasn't made any routing changes on their end If these steps don't resolve the issue, you'll need to: 1. Capture SIP logs during a failed transfer attempt 2. Check your provider's dashboard for detailed error messages 3. Verify no changes were made to your provider's routing configuration If this doesn’t resolve the issue, please let us know and we’ll be happy to step in and assist further.