"Manually add" callback requests (in the dashboard).
Receive callback request notifications via email.
Look around and see how Callback Requests works.
You can't ...
Accept callback requests on your website.
Receive callback request notifications via text message.
This free trial is all you'll need if you're after for a solution allowing receptionists to record callback requests from inbound calls. If you want to allow website visitors to request callbacks you'll need a plan. Once you sign up for the free trial you can pick a plan.