WiFi calling unreliable

I use WiFi calling on my Samsung S25 due to a weak 4G signal at home. If I leave 4G and WiFi on then it only uses WiFi calling as a last resort meaning the call breaks up a lot if the 4G signal is weak. If I put the phone in airplane mode and turn WiFi on to force it to use WiFi, then it mostly works well. Except a lot of incoming calls go straight to Voicemail, and often I can't make calls (it just doesn't dial) and I have to turn WiFi calling on/off to make it work. It was the same on my Samsung S10e.
Any idea how to improve this?

On Xiaomi phones you can specify what network to prefer WiFi or mobile.
Not sure if Samsung has same options

Unfortunately the Samsung doesn't have that option (although oddly it does with an O2 eSIM)

I have only joined Lebara today. It is so glitchy in every way. You even have to arrange the settings yourself manually if using roaming or even to use the internet. Each time. This is always done automatically by other providers. I know L is cheap but with these issue, my advice is get a PAC code and move. By the way I tried to get a PAC code and all I got back was an error message. How have you survived this long with them dude?!

Never had to enter settings when roaming. Works great for me never had to enter anything.

I got sent an email telling me I need to change my phone settings for accessing internet. Very convoluted ones too. And another one previously saying I will need to change my settings on arriving at the EU country. I’m speechless.