Welcome to the RingCentral Community

I'm unable to send SMS/MMS via the REST API at https://developers.ringcentral.com/api-reference#SMS-and-MMS-sendSMS in my sandbox environment.

I've configured my app, user & phone number successfully. I can successfully send messages with the account using https://glip-app.devtest.ringcentral.com/# and receive them both internally and externally.

I'm currently posting to: https://platform.devtest.ringcentral.com/restapi/v1.0/account/258362004/extension/677044005/sms and receiving the response below:

 {
   "errorCode": "FeatureNotAvailable",
   "message": "In order to call this API endpoint for another extension, 
               user needs to have [OutboundSMS] permission granted with 
               extended scope",    
   "errors": [ {
     "errorCode": "CMN-419",
     "message": "In order to call this API endpoint for another extension, 
                 user needs to have [OutboundSMS] permission granted with
                 extended scope",
     "permissionsName": "OutboundSMS"
   } ],
   "permissionsName": "OutboundSMS"  
 }

Here are the extension features from calling:

https://platform.devtest.ringcentral.com/restapi/v1.0/account/~/extension/677044005/phone-number?usa...

 {    
   "id": 250259005,
   "phoneNumber":"+1xxxxxxxx05",
   "paymentType":"Local",
   "location":"Warrior, AL",
   "type":"VoiceFax",
   "usageType":"DirectNumber",
   "status":"Normal",
   "country":{...},
   "features":[
     "CallerId","SmsSender","MmsSender" 
   ]
 }

I've reviewed other messages about this being an admin user trying to send on behalf of another user but this is not the case.

Any feedback would be appreciated.