WiFi-calling is not available with a dual-SIM phone while roaming

I have a European e-SIM in my iPhone 13 Pro Max as the primary SIM-card. While travelling I always buy a local physical SIM-card to avoid the expensive roaming charges. Both the SIM-cards work well expect for the WiFi-calling.

Whenever I connect to any WiFi network, I am not able to make the phone use the WiFi-call in my primary e-SIM card. The primary e-SIM always disconnects from the WiFi-call feature within seconds after connecting to any WiFi network. I can see the WiFi text briefly appear after the name of my primary e-SIM name in the top left of the display.

The only way to make or receive WiFi calls abroad via my primary e-SIM card in a WiFi network is to put the phone in airplane mode.

I would really appreciate to know whether there is any other solution to make the WiFi calls work abroad via the primary e-SIM card in a dual-SIM setup?