I have already tried with free trial keys associated with two different email addresses and a "Pay as you go" billing. They all returned this same message:
Access denied due to invalid subscription key. Make sure you are subscribed to an API you are trying to call and provide the right key.
I am using this FACE API getting started tutorial
private readonly IFaceServiceClient faceServiceClient = new FaceServiceClient("39a8b...0fd1");
var faces = await faceServiceClient.DetectAsync(imageFileStream);