How to create a new API with the second user extension on my account
Extension has nothing to do with creating an API or consuming a RingCental API.
If you are talking about consuming RingCental API in your application, then extension has nothing to do with it.
Extension only required as a parameter to pass dynamically in creating Oauth tokens.
You can pass extension dynamically here : extension=101/102/103.... so on
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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