I'm trying to setup Sinch in an app I'm making for a US-based client. The client has given me the api key, the service plan id and a from number. Trying to send a test message with a full delivery report and a callback url returns a report of a failed sms with the error code 337 which is as I can see on the error list "Invalid optional parameter".
Now, when I try the same message with my own test credentials it works, I receive the SMS and the delivery report says "delivered".
So, any idea what the problem is here?