Debug call forwarding drops

Overview

When you initiate call forwarding, you expect the call to transfer to the destination. Instead, the call drops immediately after initiating the transfer, leaving both parties disconnected.

In this guide, you’ll learn to:

• Identify why calls drop during forwarding
• Check call logs and API responses systematically
• Analyze telephony provider logs for transfer failures
• Resolve configuration issues preventing successful transfers

Prerequisites

Before you start debugging, ensure you have:

• Call ID from the dropped call
• API access to make GET requests to Vapi
• Admin access to your telephony provider dashboard (Twilio, Vonage, Telnyx)
• Wireshark installed (for SIP-based calls only)

Troubleshooting workflow

$
curl -X GET "https://api.vapi.ai/call/{call_id}" \
>
  -H "Authorization: Bearer YOUR_API_KEY" \
>
  -H "Content-Type: application/json"

1
{
2
  "id": "ca123456xxxxx",
3
  "endedReason": "assistant-forwarded-call",
4
  "status": "ended",
5
  "destination": {
6
    "type": "number",
7
    "number": "+1234567890"
8
  },
9
  "phoneCallProviderId": "twilio_call_abc123"
10
}

1
{
2
  "assistant": {
3
    "serverMessages": ["phone-call-control", "status-update"]
4
  }
5
}

1
{
2
  "phoneCallProviderBypassEnabled": false
3
}

1
{
2
  "id": "ca123456xxxxx",
3
  "endedReason": "assistant-forwarded-call",
4
  "phoneCallProviderId": "CAabc123"
5
}

$
curl -X GET "https://api.vapi.ai/call/{call_id}" \
>
  -H "Authorization: Bearer YOUR_API_KEY"

sip.Method == "REFER"

Common solutions

Based on your findings, here are the most frequent fixes:

Call shows successful forwarding but still drops

Root cause: Telephony provider couldn’t complete the transfer

Solution: Check destination number format and availability

• Verify the destination number includes country code
• Test calling the destination directly outside of Vapi
• Check if destination has call blocking enabled

endedReason is not ‘assistant-forwarded-call’

Root cause: Transfer wasn’t initiated due to configuration

Solution: Review assistant settings

• Remove "phone-call-control" from serverMessages
• Set phoneCallProviderBypassEnabled to false
• Verify your transfer function is properly configured

SIP REFER packets missing in PCAP

Root cause: Vapi didn’t send transfer request to SIP provider

Solution: Check Vapi configuration

• Verify SIP endpoint configuration
• Ensure destination format matches SIP addressing
• Check for conflicting call control settings

Limitations

Transfer scenario limitations

• Web calls cannot be transferred to phone numbers
• PSTN-to-SIP transfers are not supported
• Cross-provider transfers may have compatibility issues

Configuration dependencies

• phoneCallProviderBypassEnabled must be false for Vapi-managed transfers
• serverMessages with "phone-call-control" overrides default behavior
• SIP configuration requires proper endpoint setup

When to contact support

Escalate to support when you’ve completed all troubleshooting steps and:

• Provider logs show successful transfers but calls consistently fail
• SIP packet analysis indicates Vapi-side transfer issues
• Multiple destinations fail with the same configuration
• Configuration changes don’t resolve recurring failures

Include in your support request:

• Call ID and timestamp
• Complete troubleshooting results from this guide
• Telephony provider error codes and logs
• Screenshots of configuration settings

Next steps

Now that you can debug call forwarding drops:

• Monitor call patterns: Set up alerts for calls with unexpected endedReason values
• Test systematically: Verify transfers work across different destination types before production
• Review SIP setup: Ensure your SIP configuration follows our advanced SIP guide