Fix Azure Communication Services Integration in Next.js (React)

I'm building a Next.js + React application that uses Azure Communication Services (ACS) for PSTN and VoIP calling.
I've already deployed:
ACS resource
Azure Key Vault (secrets are stored correctly)
PostgreSQL DB
Web PubSub
Azure OpenAI
Bicep deployment is complete
However, I’m stuck at the point where ACS call automation throws this error:
Denied by the resource provider. (401)
invalid_client: AADSTS7000229 - missing service principal
I need someone to:
Fix the ACS authentication issue (likely with DefaultAzureCredential or missing service principal setup)

Validate .env.local and Key Vault secrets are loaded correctly

Ensure /api/incoming can answer PSTN calls using CallAutomationClient

Check token permissions and credentials

Make the incoming call webhook (POST /api/incoming) work properly

Tech Stack:
Frontend: Next.js (App Router, TypeScript)
Backend: Node.js API routes
Azure Services: ACS, Key Vault, Web PubSub, PostgreSQL, OpenAI, Bicep
I’ll provide:
Full project source code (via GitHub or zipped)
Access to Azure subscription + app registration if needed
Ngrok endpoint for local testing

Budget: $50–100 depending on speed and skill
Deadline: Urgent – ideally within 24–48 hours