Yep. I did some experimenting myself today with my cell phone with a slightly different result for "Busy Signal".........

Blacklist - caller hears SIT tone followed by message "Your call can not be completed as dialed. Please check the number and try again."

Disconnected - caller hears SIT tone followed by message "You have reached a number that has been disconnected or no longer in service."

Busy Signal - caller hears busy signal

Call history only showed attempted calls that were set to "Blacklisted" and "Busy Signal". The "Disconnected" calls did not show up in the call history. I always wondered why those telemarketers and political calls never showed up in the Call History

My Voipo phone never rang in any case, so I'm not sure why your busy signal function did not work correctly.