Problems sending SMS in Sandbox

Using the code below where the login works, when trying to send an SMS, I get the following exception: Parameter [from] value [**********] is invalid [Cannot find the phone number which belongs to user]

the ******** is my valid phone number. I assume this is because the phone numbers don't exist in the sandbox? How do I know what numbers to use in the sandbox? I've looked in documentation but can't find anyything.

My code:

var sdk = new SDK(appKey, appSecret, apiServer, applicationName, applicationVersion);

sdk.Platform.Login(userName, extension, password, false);

var requestBody = new {

text = text,

from = new { phoneNumber = fromPhoneNumber },

to = new object[] { new { phoneNumber = toPhoneNumber } } };

var jsonString = JsonConvert.SerializeObject(requestBody);

var request = new RingCentral.Http.Request("/restapi/v1.0/account/~/extension/~/sms", jsonString);

var response = sdk.Platform.Post(request);