If the Phonak hearing aids are switched off, it will automatically disconnect from a paired device, such as the mobile phone. Other reasons for disconnecting from the phone:
Verify your hearing aid did not run out of battery.
Ensure you are within Bluetooth range, keeping in mind that factors such as physical obstacles, interference, environmental conditions, and battery status can affect connectivity.
Make sure you have no other devices connected to your phone (such as the car Bluetooth system). This may cause your hearing aids to disconnect.