Vapi SIP Direct - 403 Invalid credentials on every INVITE Calls from our Saudi number (+966112501341) route through our on-prem PBX (Grandstream UCM6302) to Vapi SIP Direct but Vapi rejects every call with 403 Forbidden / X-Reason: Invalid credentials. Setup: - Vapi phone number: +966112501341 (type: Vapi) - Inbound SIP URL: sip:966112501341@sip.vapi.ai - Assistant linked in dashboard: Noora v6 - Naqel Express - PBX external IP: 2.88.144.75 What happens (confirmed via packet capture): 1. UCM sends INVITE sip:966112501341@sip.vapi.ai 2. Vapi replies 401 Unauthorized with Digest challenge 3. UCM retries with Authorization: Digest username="966112501341" 4. Vapi replies 403 Forbidden, X-Reason: Invalid credentials Call-ID: 9d124382-904b-4887-89d3-55c3ad054c8f Timestamp: 2026-05-22 19:34:31 Tried so far: - Public API key as password -> 403 - No SIP credential fields visible on the phone number in dashboard Questions: 1. What are the correct SIP credentials for a Vapi-type number? 2. Can you IP-allowlist 2.88.144.75 so digest auth isn't needed? 3. Should inbound calls to sip.vapi.ai even require auth, or is something misconfigured on our number?