A while back I found someone mention this in a thread and saved it. I haven't used it yet, but this may be what you're looking for. This assumes you're using the RingCentral Phone application:

1. Whilst on the active call click the transfer button
2. Enter the number you want to transfer to, and click "Warm Transfer", the first call will now be on hold.
3. Once the second call is connected, you want to bring the hold call back into the loop. Click on the phone icon in the top right of the app, then both calls will appear... click the + icon on the hold call. It will now be a conference.
4. Once the handover has taken place, Click on the green "Active Call" notification in the app, which will take you back to the keypad, and the press "Complete Transfer"

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