@Vapi I tested the transfer call tool to several phone numbers using Vapi, and it dit work really well when I was forwarding to French mobile numbers using Twilio (I'm located in France). But it didn't work at all once I wanted to transfer the call to a US phone number. Is there any explanation ?

@smozo I'm glad to hear that the call transfer worked well with French mobile numbers. Regarding the issue with transferring calls to a US phone number, there could be several explanations: 1. **International Restrictions**: Your Twilio account may have restrictions on international calling, or specific settings that need to be configured to allow calls to US numbers. 2. **Dialing Format**: Ensure that you are using the correct international dialing format for US numbers, which typically includes the country code (+1) followed by the 10-digit phone number. 3. **Twilio Balance**: Check if your Twilio account has sufficient balance to make international calls, as they may incur higher charges. 4. **Twilio Configuration**: Verify that your Twilio number is configured to make outbound calls to the US, and that there are no geographic restrictions in place. 5. **API Parameters**: If you're encountering issues with the

forwardingPhoneNumber

attribute in the API, as per the context provided, try setting it to null or an empty string to see if that resolves the issue. 6. **SIP REFER Messages**: If you're using a Vici system, ensure that it can handle SIP REFER messages, which are used for call transfers. If you've checked all these points and the issue persists, please provide any error messages you're receiving, and I can assist you further. If you need to check logs or error messages, please do so, and let me know what you find. If you're unsure how to proceed, please ask support for more info on this.