when I ask the IA to transfer the call, it just hangs up.

Hi there! I understand you're having an issue where the call hangs up when you ask the IA to transfer it. That can be frustrating! 🔍 I don't have specific details about your particular setup, but I can definitely help troubleshoot this common issue based on information I've found. One of the most common reasons for this issue is the 'phone-call-control' server event. According to [information I found](https://discord.com/channels/1211482211119796234/1211483291191083018/threads/1330092161835733013), enabling this server event can prevent Vapi from properly executing the transfer, leading to call hang-ups. The solution is to remove this event from your assistant. Other potential causes include: 1. **SIP REFER handling issues**: Vapi utilizes SIP REFER to transfer calls, so your receiving system must be configured to handle these messages properly. According to [documentation](https://discord.com/channels/1211482211119796234/1211483291191083018/threads/1333747605628391434), compatibility issues between systems can cause transfer failures. 2. **Configuration problems**: Issues with the configuration of your SIP system, network problems, or compatibility issues between Vapi's transfer method and your SIP system could be causing the hang-ups. 3. **Forwarding number format**: Ensure your

forwardingPhoneNumber

is in the correct E.164 format (e.g., +15558675309). 4. **Number restrictions**: There might be restrictions on transferring calls to certain countries - for example, you may only be able to transfer to US and Canada numbers. To troubleshoot, I recommend: - Remove the 'phone-call-control' server event from your assistant - Check your network configuration for SIP traffic - Verify your SIP system compatibility - Ensure the forwarding number is correctly formatted - Review logs for any error messages Is there any specific part of your setup you'd like me to provide more information about? Sources:

Hey! To help track down this issue, could you share: - The call ID This would really help us figure out what went wrong!