Bluetooth Doesn't Work Except on Music Channel
The car in question is a 2016 Kia Soul. The phone I'm using is an LG V20 running Android 8 (but this problem has been going on since it was running Android 6). The TomTom app will only work with the car's Bluetooth on the music channel, not the voice call, system, notification, alarm, or ring channels.
The problem with that, of course, is that any device connecting to the music channel is considered a media device and must be used instead of the car's radio, whereas the call channel can interrupt whatever is playing on the radio when there's a navigation instruction coming through. But the TomTom app doesn't work with the call channel.
At first and for several years, I figured it must be a shortcoming of the car radio and bought a separate Bluetooth speaker that clips to the visor so I could still listen to the radio. But due to the current problem with renewing the TomTom subscription, I'm taking the opportunity to test some other navigation apps, and they all work just fine using the car's call channel. So does the phone itself. It duly interrupts whatever is playing on the radio whenever a call comes through.
In other words, what I thought was a problem with the car is actually a problem with the app. It's not a deal-killing thing because I do have the external speaker, but I thought I'd let you know.