iPhone 17 pro max unreliable WiFi calling

So I have an iPhone 17 pro max. Since swapping to Lebara it’s not been able to decide whether to use the mobile network or WiFi calling at home and thus people calling me get voice mail.

my old provider didn’t do this

any solutions please as it’s driving me mad?

The work around is to manually switch off the mobile network at home

Anybody know how to flag this to tech support as it’s driving me mad. This is simple stuff