You can find the SIP URI of your application by following below steps,
Log in to your Plivo account.
Click on the Applications tab.
You will find all the applications that have been created so far. Each application name is preceded by a ‘+’ symbol, click on it to expand the application view. The SIP URI and the related URLs are visible there.
You can make an outbound call to the SIP URI of the Plivo application which you have created.For example, you can make an HTTP POST request to originate an outbound call to “sip:firstname.lastname@example.org” which is the SIP URI used to identify the Plivo application.