Your developer account credentials are different from your sandbox account user credentials.
If your developer account is created automatically for your production account, you need to enable a user in your production account to get the permission to access the developer portal via a role. See attachment.
If your developer account is a free-tier account, check you email to see if you still can find the email when you signed up. Or give me the email address you used to sign up so I can check.
Using the RingCentral Phone for Desktop, you can dial or receive test calls, send and receive test SMS or Fax messages in your sandbox environment.
Download RingCentral Phone for Desktop:
Tip: switch to the "sandbox mode" before logging in the app:
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