no call ringing when using own ntfy server

### Steps to reproduce

Hello,

I am encountering an issue with call notifications not working when using a custom ntfy server. When selecting Google Push for notifications, both call notifications and the ringtone work more or less as expected. However, switching to a custom ntfy server, the call notifications fail to come through, although message notifications continue to work without issue.

This problem persists across the following setup:
- Matrix-synapse version: v1.101.0 (Latest)
- Element version on smartphone: 1.6.10

Additionally, if relevant, a custom coturn server has been set up for handling phone and video calls.
Same problem on different phone. Had a p7 Pro with calyx + micro-G before, now having a P8Pro with GrapheneOS, same Problem again.

Could there be a compatibility issue with the custom ntfy server, or is there a specific configuration I might be missing to enable call notifications through it? Any guidance or suggestions on troubleshooting this issue would be greatly appreciated.

Thank you for your assistance.

### Outcome

#### What did you expect?

Calling notification works on external ntfy server

#### What happened instead?

No ringing at all

### Your phone model

Pixel 8 Pro

### Operating system version

Android 14 GrapheneOS latest

### Application version and app store

_No response_

### Homeserver

f-Droid 1.6.10

### Will you send logs?

Yes

### Are you willing to provide a PR?

Yes