Dual SIM iPhone sends silent SMS although iMessage and Facetime are turned off

I am using an iPhone 11 pro with an eSIM and a foreign SIM card. Whenever i activate the latter line, the phone sends a silent SMS ro the number, apparently to activate iMessage and Facetime . However, this SMS costs me quite some money as I am “abroad”. Naturally I tried switching off iMessage and Facetime for that number. Unfortunately, the phone keeps sending the silent SMS. Is there way to avoid this? Update: May this also be related to WiFi-Calling?