I have had this Panasonic KX-TGF372AZS phone for a few years now on my Fibre landline connection and has been working flawlessly, it's plugged into an extension box in another room.

KX-TGF372AZS Twin Handsets - Panasonic New Zealand

A few months ago it started saying Line in use, the phone would still have a ringtone and make and receive calls ok. So unplugged all other devices, powered off the base unit, removed batteries from portables for 30 minutes and plugged just the base station in and eventually the message disappeared to come back again the last week, doing what I did before didn't work, so I plugged it directly into the ONT and it's still the same message "Line in use"

I phoned Spark and they run some tests and said everything is fine from their end and all that I have done to troubleshoot it has to be a faulty phone, so I went out and brought another phone and that is doing the same thing, I rang Spark back and told them it's still the same after replacing the phone and must be something to do with the ONT, Chorus them can out with their fancy test phone and checked everything and said it's working fine and it was my phone that must be the problem, I told the Chorus guy that it's brand new and he said all I can do is check if the line is working and it is.

I'll take the phone over to my friends place and plug it into his ONT and see what happens

Any other suggestion ?