When you don't want to lose the call from customer so you have hold message/everyone busy running in loop. There is no way for the caller to leave voicemail before abruptly ending the call when all operators are busy attending to other callers. The below mentioned steps can help you achieve this functionality.
You can add dummy dept in the department (lets call it Support) calling it "1 voicemail".
Leave the General voice mail setting empty for the Support department.
The employee in support who normally receives the call in Support will be in the same level as Voicemail.
Then click voicemail department and set the General Voicemail setting to the email address to receive the voice messages.
Then go to Greetings for the Support department, turn on the greeting and in the text2speech you can add the following
"You have reached Support. Please hold. We will be with you as soon as possible. If you don't want to wait longer please press 1 and leave your message. We will get back to you soon as possible."
Then select voicemail from the Department's greeting drop down and ensure that this is turned off.
Ensure that you save at each step and wait for 15 minutes for the changes to take effect.