Problem Description I am trying to integrate Zadarma and VAPI to make outbound calls using a SIP URI. I have configured the SIP URI in Zadarma as lumods@sip.vapi.ai, which has been accepted correctly by Zadarma. However, when making an outbound call from Zadarma to a number in E.164 format (e.g., +351969984485), I receive the error "SIP Gateway Failed to Connect" in VAPI. Technical Details: Tests performed: I can successfully ping the sip.vapi.ai domain. Port 5060 is accessible from my network with no restrictions. The phone number is formatted in E.164, and the transport type is set to SIP. Current Status: Zadarma logs show that the call is being sent to the configured SIP URI. In VAPI, the call fails with the mentioned error. Reviews conducted: Zadarma subnets (185.45.152.0/24, 185.45.155.0/24, 195.122.19.0/27) are allowed. The assistant and phone number in VAPI have been verified to ensure they are active and correctly linked. Assistance Required: Confirm whether the domain sip.vapi.ai can properly receive traffic from Zadarma. Validate if there are any additional restrictions on Zadarma's IPs or if specific SIP headers are required for authentication. Review the VAPI configuration to ensure it can handle incoming calls from Zadarma correctly.