Since this question is nothing to do with developers or the APIs, it belongs to the customer space.
I am moving it there for better reach and hopefully you should get an answer to this
@Himanshu Patel, I would like to clarify. Where would you like the calls to be forwarded if the receiver/user is in a call?
I can recommend creating a Call Queue in simultaneous order and assigning the extensions that need to ring. Then you can configure in the user settings to go to that call queue instead of voicemail. However, it will still ring the user before connecting on the Call Queue.
https://support.ringcentral.com/article/10514-call-queue-set-up-call-handling-how-calls-transferred-group-members.html#Simultaneous