I cannot pair my PHONAK hearing aids after I upgraded to iOS 16.5 on my iPhone Xr a couple of days ago

The better question is probably how can you fix this. It is possible that PHONAK has not caught up to the latest iOS upgrade. In that case, you would want to contact them for help.

But please try this first. It may put everything back in sync.

• Delete the PHONAK app.
• Turn your hearing aids OFF completely. Power your iPhone off.
• Reinstall the PHONAK app then on your iPhone, follow these settings to set up the hearing aids again: Use hearing devices with iPhone - Apple Support

I’ve copied the pertinent information below.

Pair a hearing device with iPhone

1. Open the battery doors on your hearing devices.
2. On iPhone, go to Settings > Bluetooth, then make sure Bluetooth is turned on.
3. Go to Settings > Accessibility > Hearing Devices.
4. Close the battery doors on your hearing devices.
5. When their names appear below MFi Hearing Devices (this could take a minute), tap the names and respond to the pairing requests.
6. Pairing can take as long as 60 seconds—don’t try to stream audio or otherwise use the hearing devices until pairing is finished. When pairing is finished, you hear a series of beeps and a tone, and a checkmark appears next to the hearing devices in the Devices list.

You need to pair your devices only once (and your audiologist might do it for you). After that, your hearing devices automatically reconnect to iPhone whenever they turn on.