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I see, I tried it too; it works with the number under the sandbox account. But why can't I put another numbers from my ringcentral account? is it because the app I'm working on is not in production yet? I am a bit confused as I don't recognize the number associated with my sandbox account.

I tried it myself. I can reproduce the problem.

I resolved the issue by using the sandbox account number which could be found this way:  login developer.ringcentral.com, on top right there is a dropdown menu with your username, in the dropdown menu there is a menu item "Sandbox accounts".  Click that menu and I found "Main Company Number". Using that number as caller ID doesn't have the problem described above.

Hi Tyler, 
I am not using the extension number. I am putting the long number in e.164 format.
And I cannot post some code snippet as I am using your RingCentral Connect Platform API Explorer : https://developer.ringcentral.com/api-explorer/latest/index.html

I have created an application that is not yet in production and I am testing it using your API explorer platform. When making a ringout call through this app, it works perfectly when the calling ID is empty. When I put the 11 digits number in the calling ID field in E.164 format (for instance: +14445559999), then I get the error.

Direct number is not extension number. The former is normally longer than the latter. Please double check.

 If it still doesn't work, could you please post some some code snippet?