Poor coonection during landline calls

[Danno]

I have been having issues with calling landlines. I can usually hear the person on the other end but they can't hear me. This issue is only a calls to landlines problem as calling mobiles is always fine. 

This was happening on my previous sim and phone and I expected an upgrade to resolve the issue with a new sim card and new number, unfortunately the problem still occurs. 

I've called Three and they're unsure of the issue. 

[Danno]

I've tried turning the phone on/off, I've manually selected on the sim settings Three; and the area checker is showing as no faults in the area. 

The only thing I haven't done is turning off wifi calling, however I've called landlines when I'm not connected to WiFi anf had similar issues.