OVH firewall settings (VOIP)

Hello,

I've installed OpenVPN on a VPS with Debian 10 (TCP 443).
I can successfully use it when I'm connected to a Wifi hotspot from my Android smartphone even if OVH firewall is ON.
But I can't make any phone or video call with Whatsapp if OVH firewall is ON: the destination device rings but communication is never established after picking up (timeout after 30 s.).
All is fine if OVH firewall is OFF ! So, it seems to be the culprit...

It's very strange because according to official doc, only 2 **outgoing** ports are required (443, 5222).
whatsapp/guides/network-requirements

OVH firewall only handles **incoming** traffic, doesn't it ?
It's a huge mystery to me !

In OVH firewall rules, I authorise 2 ports :
- TCP 443 for OpenVPN
- TCP 9999 for SSH (direct access in case of VPN failure)

What could be the problem ?
I plan to sniff Whatsapp network traffic from my Android phone but I hope I'm not the first to try to use Whatsapp with OVH firewall & OpenVPN and someone has a solution ! ;)

Thanks in advance !

Stéph.