Phonak hearing aids (as well as the Kirkland Signature models sold at Costco) do not use the "Made for iPhone" (abbreviated as MFi) Bluetooth protocol.
The Made for iPhone Bluetooth protocol was developed by Apple working with major hearing aid manufacturers. With the exception of Phonak (along with the other brands in the Sonova family), all of the major hearing aid manufacturers produce MFi-compatible hearing aids.
One of the primary benefits of the Made for iPhone Bluetooth protocol is that it is a low-latency connection. Latency is the amount of time it takes for the audio to travel from the iPhone to your hearing aids.
Phonak hearing aids use the traditional Bluetooth standard which has a higher latency, so it takes more time for the audio to go from the iPhone to your hearing aids.
Why this matters
In an audio-only scenario (like talking a phone call or listening to music), latency really doesn't matter. However, latency matters a great deal if you can see the person talking. Imagine seeing a person talk but having their voice delayed. If this happened while you were watching a video, it'd be annoying. If the latency (delay) was small enough, maybe you could live with it. But at some point it would be extremely distracting, perhaps to the point of being unwatchable.
This can also be true for in-person conversations. A delay between seeing someone talk and hearing what they say can be distracting.
The Made for iPhone Bluetooth protocol has the lowest latency currently available. The higher audio latency the in standard the Bluetooth protocol (which Phonak hearing aids use) would result in an unsatisfactory experience.