You cannot guarantee that users will read text.

Assuming you put it front and center, you can help them notice the message if you keep the text short and to the point. More text is dilution. People scan, they don't read.

A/B testing doesn't lend itself to all decisions but it's perfect for copy testing. If you wanted to compare 2 writing approaches to see which is more effective at getting users to notice or comprehend certain ideas, try a few experiments with memory tests.

Many ways to test assumptions, this is just one.

Editing my answer, since the question has since been sharpened.

These are mandatory work instructions. Such as it is, the job of design here is to convey that to the user. Reflect that in the visual hierarchy:

(First element in order, and the most visually prominent:)

"Read the following 3 bullet points aloud to the customer on the phone:"

(Second element in order:)

"Bullet 1" "Bullet 2" "Bullet 3"

I echo whomever suggested a 2-step wizard in a modal window. The best way to increase the user's focus is to take away all competing distractions and force the user to consume the content in the order you dictate.

Think of a those awful pre-roll video ads. We hate them because we have to view them in order to view what follows. That stepwise model is actually useful here.