No warning if voice call cannot be established because of unverified session existing somewhere

### Steps to reproduce

1. I make a voice call from a verified session on my handled device.
2. Everything seems to be working properly, except that I don't hear a ringtone.
3. The call never reaches the other party, for no apparent reason.

Note: I have some unverified sessions on my PC.

### Outcome

#### What did you expect
A warning should be displayed like "This voice call cannot be established because you have unverified sessions". A similar warning is displayed when a text message is sent.

#### What happened instead?
Nothing

I understand the security concern if a communication is established if unverified sessions are alive somewhere. But a warning should appear in the voice call interface as it is in the text interface.

### Your phone model

Pixel 8

### Operating system version

Graphene OS

### Application version and app store

26.06.2 F-droid

### Homeserver

matrix.org

### Will you send logs?

No

### Are you willing to provide a PR?

No

The report identifies the voice-call interface and the existing text-message warning as the relevant entry points, but names no files or tests. Trace how unverified sessions are handled for text messages, compare the voice-call failure path, and define done as showing an equivalent warning when the call cannot be established.

Written by the indexing model from the issue text.