You can program an Asterisk queue so that when people tire of waiting, they press a digit and get to a menu where they can leave their number. Then the system queues their call and attempts to call them at a convenient time.
This is possible using the new WombatDialer predictive dialer for Asterisk call center.
Read the tutorial for free at
https://www.wombatdialer.com/blog/blog/2018/02/26/queue-call-back/?lid=U068
