Cannot connect voice calls when using cellular data

[pranavmishra90]

Describe the bug
Users on the same matrix server on iOS cannot receive voice calls when using cellular data. WiFi connected devices function normally.

To Reproduce

Element Desktop/Web call to iOS on Wifi

Works as expected

Element Desktop/Web video call to iOS on Wifi

Works as expected

Element Desktop/Web call to iOS on 4G cellular data

1. iPhone receives incoming call notification
2. Answer call on phone
3. Element Desktop reports that the call has been joined by iOS user
4. Element iOS app freezes. The call cannot be hung up. The only button which "works" is switching the audio device. All other buttons do nothing.
5. App requires to be force closed and reopened

All iOS ---> Element

Tests have the same results as above

iOS on 4G <---> Element Web on a federated server (matrix.org)

Interestingly enough, when connecting to a user on another server, the audio call works as expected, even on cellular data. To be clear, this is between one user one user on matrix.org calling a second on my personal server.

Expected behavior
I expect that voice calls between users on the same matrix server should be able to connect regardless of whether the iOS users are on cellular data or wifi

Screenshots

(Note that this "Reminder Bot" is not actually a bot, but a user account which I am testing from)

Desktop (please complete the following information):

• OS: Windows 10
• Browser: Brave
• Version: 1.24.85 Chromium: 90.0.4430.212 64 bit

Also using Element Desktop for Windows

• Version (previously latest stable, now 0.0.1-nightly.2021051301)

Smartphone (please complete the following information):

• Device: iPhone Xs
• OS: iOS 14.4.2
• Browser stock browser

Additional context
Using the spantaleev/matrix-docker-ansible-deploy playbook for matrix setup. Server is sitting behind a reverse proxy.

Matrix Synapse server_version: 1.33.2
Python: 3.8.10
Coturn enabled: +/- (tried both)
Fallback to matrix.org TURN +/- (tried both)

Not sure if it is related to #3054 or not?

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Thanks for your time

[pranavmishra90]

Possibly relevant section from the Matrix.org website's FAQ section:

Do I need a TURN server?

VoIP calls should work if both parties are on public networks. However, in practice one (or both) devices are often behind NAT, and so having a TURN server is important to help set up the call.

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I have tried to use my own TURN server (via the ansible playbook) and the Matrix.org fallback. Still having the error

[mel-geo]

I’m experiencing the same problem