How do I set up a busy message?

When I am on the phone and another call is coming in how do I setup a message to say "I am currently on the phone please leave a message".

Here’s how:

When there’s an incoming call, you can tap incoming phone icon and drag it up to quick response icon(messages).
Next, you can tap a pre-typed text or select Write your own… to quickly type a custom message to respond to the call.
If you wish to customize the pre-typed messages, follow these steps:

On your Android phone, find and open the Phone app.
Tap the Menu button > Settings > General settings > Quick responses
Tap the pre-typed text you want to change.
Enter a new message, and then tap OK.