Hello, I’m Nathan, and I’ll be assisting you today. My livelihood depends on your positive feedback, so your satisfaction is my highest priority.
Sorry to hear you’re having trouble. We’ll get started chatting here. I am also sending you an offer so you can reach me directly me by phone if preferred. Feel free to ignore it for now, it will be available to you at any time if you need to get more help after our chat.
Unfortunately, as phone technology goes, yours is quite an old model (from 2007). There are no longer any parts available for your system, including replacement handsets.
I'm assuming you have the KX-TG8120 base station. The TGA810 is the replacement handset that was sold when this system was new. It was discontinued in 2009. It was replaced briefly by the TGA820, though that has also been discontinued many years ago.
These are unfortunately the only handsets that are compatible with your existing base station, and you won't find any of them sold currently, at least not new. You could try your luck watching for a used one on ebay, but these phones just don't have that long a life span, so it wouldn't be worth the investment.
Additionally, even if you could find the part, it will run you about £40 just for an additional handset. You could buy a brand new phone system with 3-5 handsets for £50-100.
If you want to try that route, this is the model I'd recommend for you:
Panasonic KX-TG8063EB Triple Colour DECT Phone Set with Answer Machine (click for link)
I'm sorry there isn't an easier option here. You'll find this is typical of all cordless phone manufacturers - handsets are only compatible with certain base models, and parts are only made for a year or 2 before the system is replaced with incompatible new systems. In almost all cases, it's cheaper to start fresh with a new system than trying to find parts to fit the old, and you get the added benefit of new features and a fresh warranty