I don’t use Phonak aids but I do have an 8851 at work (Cisco phone system at a University).
My Oticon aids (OPN when I got the phone) use MFI rather than normal Bluetooth so I have to use a ConnectClip as an intermediary device between the Classic Bluetooth of the 8851 and the MFI of the Oticon aids. The ConnectClip has a microphone. The combination of the ConnectClip and the Oticon aids are seen as a Bluetooth “headset” by the 8851. Your aids may be seen as a headset also. In my case I paired the ConnectClip with the 8851. When powered on, the ConnectClip connects to the first paired Bluetooth device it finds so that may be my computer (MacBook Pro) or the 8851 phone. If the computer, I disconnect it using the computer Bluetooth control and use the phone setup menu to connect to the “headset”. After that I can use the ConnectClip or the phone buttons to answer the 8851. As you won’t have the intermediary device, maybe the 8851 will connect to you aids automatically if they aren’t connected to another device (computer or cell phone or whatever). I’ve read that some Phonak aids can pair with multiple devices but connect to either 1 or 2 bluetooth devices depending on the model of the aids.
I hope this helps a bit as I didn’t find much info when trying to use 8851 with my aids. You may find it useful to download the complete users manual for the 8851 if you don’t get one with the phone. It has some info about how to use Bluetooth
Good luck with your new phone.
Stuart