First, in your script, add a custom string variable and make sure the input toggle is set to true. Display the variable in a read-only text component. Save and publish the script.
Next, open your flow and find your set screen pop action. It will show the input variable you just created. Make sure you are in expression model instead of literal, and assign it the value Call.CalledAddress. This is a default call variable and should show up in the IntelliSense while you are typing, otherwise you are in literal mode. Publish the flow.
Now when an agent gets a call and the script opens, it will show the number the customer dialed.
Documentation resources:
https://help.mypurecloud.com/articles/add-script-variable/
https://help.mypurecloud.com/articles/read-text/
https://help.mypurecloud.com/articles/set-screen-pop-data-action/
https://help.mypurecloud.com/articles/use-expression-editor/