There are different SDKs available for RingCentral.
Which one supports of iOS? Not sure Java or PHP supports.
Long time ago I wrote this tutorial. It might help you to get started with the iOS SDK.
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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