I believe this is because there is no digital line associated to this user. 

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Hi, Patel. If you are using sandbox environment, you can add free digital line(phone) to your extension in here http://service.devtest.ringcentral.com/ . Need to login with your sandbox account. 
For RingCentral Embeddable, you can find document and examples in here https://github.com/ringcentral/ringcentral-embeddable

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