I have the same issue on devices in my home wifi network also.
It is marked something like "connection in progress" and its on Android 10 & 12.
Don't know what could be the issue... @rares your firewall seems to be default configuration, isn't it ?
Do you have Secure DNS on your android devices ? If yes, maybe try without it ?
Is your Android a custom rom ?
On WhatsApp enable "the Nearby Devices-permission" if it's listed. And test again.
Also here is a list of changes you can make-
Set WhatsApp battery to unrestricted
Turn off background usage limits > put unused apps to sleep
WhatsApp > application data usage > allow background data usage turned ON
WhatsApp > application data usage > allow data usage while data saver in on turned ON
Intelligent WiFi > switch to mobile data turned OFF
I tend to believe the issue is with Whatsapp only & not OpenWrt.
If everything fails then reset your firmware to default settings and test again
@rares If I'm correct, your issue is the same as me ?
When a call comes or go, it's okay for the reception. But as soon as we answer the call or they answer the call, we have on the phone that the "connection is in progress" (in other words). The connection never establish.
Moreover, I've tried on another network with the same Internet box & provider but without OpenWRT router behind and it works. Both phones connected to the WiFi have the same behavior on both networks.
Because of those reason, I doubt it's an issue on the phones.
I'll try asap on WiFi network of the internet provider box directly to confirm.
The description in this comment is very accurate of the situation.
Yes, I have tried with Adblock disabled and it did not make a difference.
Does their phone continue to ring despite them pressing answer (signalling not being generated/relayed correctly)?
No. As soon as they answer the phone goes into Media mode. There is a period of 1-2 seconds where it tries to start the call (see/hear the other party) and then it disconnects. The WhatsApp window shows an error message about the WiFi network not allowing calls.
Does the call drop immediately when they answer (possible codec issue, but that seems unlikely given you're both using Whatsapp)?
No, see previous answer.
Is the call established (incrementing call timer) but with silence for one or both participants (routing issue leading to packet drops)?