ASUS - ZenTalk

I can confirm all of this. What can I add:
it happens no matter what version of Android we use (I even downgraded to Andoid 11 with clean image)
it happens when both phones using VoLTE - when my interlocutor uses 3G everythings works fine, despite im connecting using LTE, so that doesn't look like connection problem
it hapens also on VoWiFi
it happens only to Zenfone 8 - i know people with Realme and Samsung in the same carrier - their VoLTE works perfectly
I think it's problem with some codec support on Zenfone 8. T-Mobile (and 3 of 4 big carriers in Poland) uses EVS SWB in VoLTE, while classic HD Voice in 3G calls uses AMR-WB.
I checked on this site: WiFi Calling, VoLTE i EVS w Polsce (wificalling-volte.pl), there is Rog Phone 5 on supporting EVS list, but no Zenfone 8.

You wrote:
"it happens when both phones using VoLTE - when my interlocutor uses 3G everythings works fine, despite im connecting using LTE, so that doesn't look like connection problem
 
I'm facing the problem even when someone doesn't have voLTE - only on outgoing calls - incoming calls work perfect 😞

Later I'll check incoming calls yet, and give information.