When you say sometimes it does not work, you need to give the full details on what exact issues you are actually facing when you have implemented it for missed call event .
Some more details or any exception details will be helpful to understand the issue
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:
2 People are following this question.