Receiving a CMN-211 / HTTP 503 obtaining an OAuth token from DevTest environment

When I tried to obtain an access_token from the DevTest/sandbox environment I received a HTTP 503 response / CMN-211 response instead of the expected access_token and a 200 response for the following request:

POST /restapi/v1.0/oauth/token

HOST: https://platform.devtest.ringcentral.com

HTTP Headers:

Content-Type: application/x-www-form-urlencoded;charset=UTF-8
Authorization: Basic <BASE64_ENCODED_API_KEYS>
Accept: application/json

POST Body:

grant_type=password&username=<RC_USERNAME>&password=<RC_PASSWORD>&extension=<RC_EXTENSION>

NOTE: I did replace the values in tags above with actual values

The response I received was:

{ 
  "errorCode": "MaintenanceMode",
  "message": "Service is overloaded, please retry later",
  "errors": [
    {
      "errorCode": "CMN-211",
      "message": "Service is overloaded, please retry later"
    }
  ]
}

Is the devtest environment in Maintenance mode, or am I doing something incorrectly?

question