You should be able to add any DECT compatible handset to your existing base station, you may have to un pair the existing handsets first tho, depending on how many handsets the base station will handle.
Whats gone wrong with the original handsets?
Had a new line, so thought I would have the new phones.
Engineer had trouble with the line when installing due to and unbalanced line and the install took 4 hours.
After that I had issues with an echo on the phone that seems to disapear after a min onr so. I also noticed this plus poor volume issues seemed to be happening when it was raining/poor weather.
Put at least 4 complaints into BT faults but seemed to improve after a line test. The guy at the test centre suggested it may have been water on/in a joint and the fact they send a small voltage down the line some times dries it out (His explanation - and if wrong please dont shoot the messenger.

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Any way, tues I was in the middle of a call I I suffered from massive feed back. Had to end the call. Phoned faults for a test and was told the line was fine but we could hardly hear each other, he rang me back on my mobile. He asked if Ihad a fax machine, to which is a no, both hansets were the same problem. So he booked a repair and warned me if no fault found I was liable for a bill of £125.
Now with the install issues and the fault repoarts I honestly thought it was a BT issue.
Anyway, nipped and got my old phones and guess what, no problems with feed back. so it seems the issue is the base unit/charger.