I think I understand....
If people are on the line, the next call gets a "busy" tone.
There may be a couple of things you can do.
1- set up a special hunt groups of keys on all the phones. Then if the person you called is on the line you can forward to those keys. You can can also add a rad to this group so that if people don't answer they can hear the rad that says "everyone is busy, please hold" and then play
music on hold. I've used this set up before - on a few systems - going all the way back to when the SX2000 light was introduced. But... There is an issue with the buttons locking up. Never been able to prove the cause and Mitel says I'm full of crap but it takes a reboot to resolve.
2- you could forward your calls on a busy to a ring group with multiple phones. Can't do a rad to music on hold but you can get it answered.
3- forward your calls to VM.
Hopefully that helps.
Ralph