Why would it thrill the supporters to listen in on the phone calls and read the letters?
It would thrill supporters because they would see firsthand that God is touching a lot of hearts through the ministry they helped fund. Listening to phone calls and reading letters would reveal that their financial support is helping reach a multitude of people with the truth of God's Word.
Les explains that he does not do this for monetary gain or to become famous. His goal is to reach a multitude of people with the truth of God's Word, and God is blessing it. He wishes especially the class people who helped financially could listen in on the phone calls and read the letters, because it would thrill their hearts to see that God is touching a lot of hearts.