Modifying phonatory intervals requires biofeedback about your speech. It is also relatively new so hence there’s not much info about it. In order to do that therapy you would need to get the app and also employ a speech pathologist trained in that approach. You can do that via their app (search for MPI-2 on the App Store). It is expensive though ($900 for the app alone). There’s much more info on prolonged speech online. Search YouTube for Camperdown Stuttering as this program uses prolonged speech. Camperdown Treatment manual is available here: https://sydney.edu.au/health-sciences/asrc/docs/Camperdown%20Program%20Treatment%20Guide%2024%20Feb%202017.pdf