Maybe not in the way you are hoping. If a call was handled by a voicemail system that supported "calling party busy" message greetings than you can inform the caller that the reason they were redirected to voicemail was because the original called party was busy. Another option would be to leverage BLF for directories (Enterprise Parameter) and have users change their behavior to check the corporate directory/missed calls/received calls/etc. to get line status info before placing the call. Whether you have access to this feature or not would depend on your CUCM version.
If you are looking for a status message or custom ringback tone to indicate that the remote party is on the line, this is not possible.
HTH.
Regards,
Bill
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