Phonak "Preferred Providers"

When searching for Phonak hearing aids they list some that are “Preferred Providers.” (I know there is an archived thread on the topic but it didn’t really answer the question) What is required to become a “Preferred Provider.” I can conjecture myself so hopefully will hear from somebody who actually knows @Michael_Phonak can you comment? I’d also be interested to hear from @Neville and @Um_bongo