Hi - no, when the other person calls me I see nothing, they just get the busy tone. But like I say, not sure if that is because I've not answered the call on the landline, it's technically still ringing. We don't own a physical phone to plug into the landline any more. The line is purely there for broadband only, if I didn't have to have it then I wouldn't.
I've never used the eSim with anyone. Forgot all about that! I can't remember who I was with when I bought the phone, but it was bought direct from Apple unlocked and it's always been SIM cards.
I suppose all I can really do is a) wait until I know I've missed another important call (or it came up and notified me!) or find a 3rd party I can get involved with testing properly.
[EDIT] PS: My wife has been on giffgaff for donkeys years (I left and came back) and says call wiating used to work on her iPhone XR but she's noticed it doesn't any more either so she isn't sure if it's a botched software thing. I can't take that as a 100% reliable comment because I know 99% of her calls are over WhatsApp