Quick Start SMS -- SendingFailed

I followed the instructions for the Quick Start App to send SMS in Python. Using the Sandbox credentials, the sms queues, but isn't delivered. I have confirmed that number has the 'SmsSender' feature. I have tried sending an sms to two personal mobile numbers as well as another RingCentral number that I know is able to send/receive sms. The status printed back is 'Queued'; when I check the messageStatus, they all show 'SendingFailed.' I'm not sure what I'm missing! Any help would be appreciated.

After the phone number info, the typical message info is as below:

{
   "type":"SMS",
   "creationTime":"2021-07-01T17:38:30.000Z",
   "readStatus":"Read",
   "priority":"Normal",
   "attachments":[
      {
         "id":9819398004,
         "uri":"https://platform.devtest.ringcentral.com/restapi/v1.0/account/299110004/extension/299110004/message-store/9819398004/content/9819398004",
         "type":"Text",
         "contentType":"text/plain"
      }
   ],
   "direction":"Outbound",
   "availability":"Alive",
   "subject":"Test SMS using a RingCentral Developer account - Hallo World.",
   "messageStatus":"SendingFailed",
   "smsSendingAttemptsCount":1,
   "conversationId":3060223407456103798,
   "conversation":{
      "id":"3060223407456103798",
      "uri":"https://platform.devtest.ringcentral.com/restapi/v1.0/conversation/3060223407456103798"
   }
}