Hi there! Sorry to hear you're experiencing issues with outbound calls. 🔍 I can definitely help you troubleshoot that "sip telephony provider failed call" error you're seeing with your Vapi implementation. Based on the information I found, here are several potential fixes for this issue: ### Check your SIP configuration - Verify that your SIP URI in the

sipUri

attribute within the

forwardingPhoneNumbers

array is correctly formatted and points to a valid SIP address. [According to Vapi documentation](https://discord.com/channels/1211482211119796234/1211483291191083018/threads/1336713212233515089), this is a common cause of the error. - Ensure your SIP server or PBX is configured to accept calls from Vapi's servers and isn't rejecting incoming INVITE requests. ### Network configuration - Make sure your network allows traffic on the SIP protocol and that any firewalls or NATs are configured to allow SIP messages to pass through. - Check if there are any blocks on the ports used by SIP in your firewall settings. ### Authentication issues - If your SIP endpoint requires authentication, double-check that the credentials provided to Vapi are correct. - Vapi expects authentication credentials in the Authorization header, including the username, realm, nonce, and URI. [The username must be your SIP username](https://discord.com/channels/1211482211119796234/1211483291191083018/threads/1334238882660618250), and the password can be anything except "test". ### SIP REFER handling - Confirm that your system is set up to handle SIP REFER messages, as [Vapi uses SIP REFER to transfer calls](https://discord.com/channels/1211482211119796234/1211483291191083018/threads/1336713212233515089). ### Codec compatibility - Ensure that the codecs used by Vapi are supported by your SIP endpoint. ### Check for limitations

checking if this is resolved/solved for you?