Hi Huy,

Sadly this feature is not available in CME or CCM. The closest thing in either system is called Callback. This only works for Internal Extensions;

Callback-Requests callback notification when a busy called line becomes free.

Callback Busy Subscriber

This feature allows callers who dial a busy extension number to request a callback from the system when the called number is available. Callers can also request callbacks for extensions that do not answer, and the system will notify them after the called phone is next used.

There can be only one callback request pending against a particular extension number, although a caller can initiate more than one callback to different numbers. If a caller attempts to place a callback request on a number that already has a pending callback request, the caller hears a fast-busy tone. If the called number has call forwarding enabled, the callback request is placed against the final destination number.

No configuration is required for this feature. To display a list of phones that have pending callback requests, use the show ephone-dn callback command.

http://www.cisco.com/en/US/docs/voice_ip_comm/cucme/admin/configuration/guide/cmecover.html#wpmkr1128179

Hope this helps!

Rob