Custom responses allows you to reply to a caller simply by letting Yodel read out your personalised message.
Once you activate the feature canned responses in by adding a pre-defined reply, you will also get the option in Slack to write in your own reply.

Canned Responses Feature — Yodel

Once a call comes in, you will have the option to respond with a canned response or to type in your own reply.

Respond with a Custom Response in Slack

By clicking on "custom response", a white text field pops up and you can write in your reply.

Write a Custom Response — Yodel

After hitting "Send", Yodel will read out your message to the caller.

Yodel — Slack Integration

Note: Please be aware that you can only send a custom response once.