Your extension is not allowed to make outbound calls with browser currently, please contact your account representative for an upgrade.

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