Saladin,
Lines in CallManager have 3 possible states: idle, available, busy. When User1 dials User2, User2 is available. So, when User1 dials User2, there is no feature built into CM that will allow you to play a custom ring back to the caller being that the called party is on an active call (available state). When a line has no active calls, its idle. When it has as much active calls as configured in the busy trigger, it is busy, and instead of a ring tone, the caller hears a busy signal.
Probably your need translates with different word into this: Presence. With Presence services, your configured callers can monitor from their phones if other lines are on a call or idle (idle state). They would just need to go to the Directories and see what is the status of the other parties, and then attempt a call right away or wait a little:
Check this link for more detailed information, I think it will provide what you need here, it is just that in another way:
Introducing Presence
Configuration Checklist
They refer to directories when talking about call lists, just in case.
Another feature you may want to look into, is the Callback feature:
"To receive call-back notification, a user presses the CallBack softkey while receiving a busy or ringback tone. A user can also activate call back during reorder tone, which is triggered when the no answer timer expires."
http://www.cisco.com/en/US/partner/docs/voice_ip_comm/cucm/admin/5_1_3/ccmfeat/fscallbk.html#wp1026410
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