the output sound of a call is not always 'call' it is sometime 'media'

### Steps to reproduce

1. Where are you starting? What can you see?
I do a call between 2 mobile apps (but it also happens when one is web app).

3. What do you click?
I click on the volume buttons of the phone.

### Outcome

#### What did you expect?
I expect that the output is 'call' and that i can use the volume buttons of the phone.

#### What happened instead?
Sometimes the output is 'call' and sometimes it is 'media'. If it is media I can not use the volume buttons.

### Your phone model

Google pixel 8a

### Operating system version

Android 16

### Application version and app store

Element X 25.06.02

### Homeserver

matrix

### Will you send logs?

No

### Are you willing to provide a PR?

No

No source files or tests are named. First reproduce the audio-routing behavior on a Pixel 8a running Android 16 with Element X 25.06.02, then trace the call audio output selection and volume-button behavior; done means call audio consistently uses the call output and the phone volume buttons control it.

Written by the indexing model from the issue text.