Microphone audio stops transmitting after declining an incoming cellular or FaceTime call

### Steps to reproduce

1. Start a voice (or video) call with another user in Element X on iOS.
2. While the Element X call is ongoing, receive an incoming cellular or FaceTime call.
3. Decline the incoming call.
4. Continue speaking in the Element X call.

### Outcome

#### What did you expect?

After declining the incoming cellular or FaceTime call, the Element X call should continue normally and my microphone audio should still be transmitted to the other participant.

#### What happened instead?

As soon as the incoming cellular or FaceTime call appears, my microphone audio stops being transmitted in the Element X call.

Declining the incoming call does not restore the microphone audio. The Element X call remains active, but the other participant can no longer hear me.

### Your phone model

iPhone 16

### Operating system version

iOS 26.5.2

### Application version

26.07.4 (236)

### Homeserver

Synapse 1.156.0

### Will you send logs?

No

No files or tests are identified in the issue. First reproduce the interruption on an iPhone during an Element X call, then trace the relevant call-audio handling; done means the remote participant continues receiving microphone audio after the cellular or FaceTime call is declined.

Written by the indexing model from the issue text.