sending to voicemail instead of ringing second extension when on an active call

I am using ring central with several extensions. When a call comes in it goes to my desk extension, then if I don't answer it rings my cell phone extension. But it also goes to my cell phone extension if I am on an active call on the desk extension. I would like to see a setting that when I am on an active call it can be directed to voicemail instead of ringing the cell phone extension. It is very annoying to be on a call and my cell phone rings and rings as hitting the "X" to decline just makes it ring again a few seconds later.