Requiring a phone number may be in breach of existing legislation, particularly that which resulted from the No vote to the Australia Card in the 80's. Those anti-id laws were ratified in 2005/2006.

https://www.aph.gov.au/About_Parliament/Parliamentary_Depart...

This tech can and should be using crypto random hashes rather than phone numbers. The authorities don't need to call anyone, let the app and the fully anonymous central db do it's job to notify others that they need to get tested.