POST /api/v2/conversations/calls returns 400 Bad Request with 'invalid phone number'

Trying to initiate an outbound call on behalf of an agent using the .NET SDK. Sending a valid E.164 number in the JSON payload but getting a 400 error with ‘invalid phone number’ in the response. The same number works fine in the standard desktop app. Checked the formatting multiple times. Any idea what I’m missing?