4. **Web Call Limitations**: Call transfers are not supported on web calls and will result in a hang-up. [Documentation shows](https://discord.com/channels/1211482211119796234/1211483291191083018/threads/1337162299621900359) that using the transfer calls functionality requires phone calls instead of web calls. ## Troubleshooting Steps Here are some steps you can take to troubleshoot: 1. **Check for 'phone-call-control' Event**: Remove the 'phone-call-control' server event from your assistant if it's enabled. 2. **Verify Phone Number Format**: Ensure your forwarding number is in the correct E.164 format. 3. **Check SIP Configuration**: If using SIP, confirm your system can handle SIP REFER messages. 4. **Review Call Logs**: You can view logs for your specific Call IDs by going to the Vapi Dashboard, navigating to "logs > calls", or directly accessing them via

https://dashboard.vapi.ai/calls/{call_id}

. 5. **Try Using a Twilio Number**: [Some users reported](https://discord.com/channels/1211482211119796234/1211483291191083018/threads/1339690911675977838) that using a number from Twilio resolved their issues with the

transferCall

function. Is there anything specific about your setup that might help narrow down the cause? I'd be happy to provide more targeted troubleshooting steps. Sources: - [Discord Channel phone-call-control Server Event](https://discord.com/channels/1211482211119796234/1211483291191083018/threads/1330092161835733013) - [Discord Channel SIP REFER Issues](https://discord.com/channels/1211482211119796234/1211483291191083018/threads/1337383511329345588) - [Discord Channel Forwarding Number Format](https://discord.com/channels/1211482211119796234/1211483291191083018/threads/1335869152535121960) - [Discord Channel Web Call Limitations](https://discord.com/channels/1211482211119796234/1211483291191083018/threads/1337162299621900359)

- [Discord Channel Twilio Number Solution](https://discord.com/channels/1211482211119796234/1211483291191083018/threads/1339690911675977838)

Hi there! 👋 I understand you're experiencing an issue where assistants are hanging up instead of transferring calls. While I don't have specific information about the exact call IDs you mentioned, I can definitely help with this common issue. This is a known issue in Vapi where assistants hang up instead of transferring calls. Let me share what might be happening and how to fix it: ## Potential Causes According to the information I found, there are several possible reasons for this behavior: 1. **Phone-Call-Control Server Event**: If the "phone-call-control server event" is enabled on your assistant, calls might be marked as forwarded but not actually transferred. [This can cause calls to hang up instead of transferring](https://discord.com/channels/1211482211119796234/1211483291191083018/threads/1330092161835733013). 2. **Assistant Configuration Issues**: A misconfiguration in your assistant's settings, particularly related to the

forwardingPhoneNumber

or conditions triggering the

transferCall

function, could be causing the problem. [Some users have reported this issue](https://discord.com/channels/1211482211119796234/1211483291191083018/threads/1334191012620402730). 3. **Model and Prompt Issues**: The model temperature and prompt configuration might need adjustment to properly handle call transfers. ## Troubleshooting Steps Here are some steps you can take to resolve this issue: 1. **Remove the Phone-Call-Control Server Event**: Try removing the "phone-call-control server event" from your assistant configuration. [This has resolved the issue for many users](https://discord.com/channels/1211482211119796234/1211483291191083018/threads/1335869152535121960). 2. **Check Forwarding Number Format**: Ensure your

forwardingPhoneNumber

is in the correct E.164 format (e.g., +15558675309). [Incorrect formatting can cause transfer failures](https://discord.com/channels/1211482211119796234/1211483291191083018/threads/1335869152535121960).

3. **Adjust Model Settings**: Try setting the model temperature to 0.3 and reworking the prompt using the Vapi prompting guide for tools calling. [This has helped some users](https://discord.com/channels/1211482211119796234/1211483291191083018/threads/1341953566801264690). 4. **Check Twilio Routing**: If you're using Twilio, try changing the routing region to US. [This has resolved issues for users in certain regions](https://discord.com/channels/1211482211119796234/1211483291191083018/threads/1339614362742165606). 5. **Try a Different Model**: Some users have found success by changing to GPT4o for call transfers. [This might improve transfer reliability](https://discord.com/channels/1211482211119796234/1211483291191083018/threads/1339614362742165606). Would you like me to provide more specific information about any of these troubleshooting steps or explain other potential solutions? Sources: - [Discord conversation about phone-call-control server event](https://discord.com/channels/1211482211119796234/1211483291191083018/threads/1330092161835733013) - [Discord conversation about assistant configuration issues](https://discord.com/channels/1211482211119796234/1211483291191083018/threads/1334191012620402730) - [Discord conversation about forwarding number format](https://discord.com/channels/1211482211119796234/1211483291191083018/threads/1335869152535121960) - [Discord conversation about model temperature and prompt adjustments](https://discord.com/channels/1211482211119796234/1211483291191083018/threads/1341953566801264690) - [Discord conversation about Twilio routing and model changes](https://discord.com/channels/1211482211119796234/1211483291191083018/threads/1339614362742165606)

#solved