That's not good at all have you spoken to the BT8500 Helpdesk? 0800 145 6789 They should be able to office advice on this.
BT8500 - a phone I love (feature set) and absolutely hate (shocking reliability). I feel the pain of fellow owners who have experienced reliability problems with this BT product.
In April 2015, I bought a BT8500 quad pack from Amazon (sold and supplied via LiGo). It lasted 4 days before the base unit died. Tried the standard support suggestions to no success. I spoke to BT support, who didn't bat an eyelid, and they simply gave me a fault code then instructed me to return it to Amazon/LiGo.
In May 2015, I bought a BT8500 trio pack from Amazon (this time sold and supplied direct from Amazon). I've had it running for over a week and suddenly all phones have lost connection to the base and won't reconnect. I've tried all known support suggestions (check cables, power down for a time, re-seat batteries, disconnect and re-attach cables etc) and nothing has fixed the issue.
My home environment (power, sockets, surge protected adapters) works perfectly fine so no issues there and my previous cordless phone still works a treat. So, I'm left with the view that the BT8500 product set has stunning reliability problems or that Amazon and it's co-sellers have received badly manufactured batches.
Oh well, I have go kick off another returns process with Amazon. While I'm contemplating a "3rd try" with the BT8500 I'm not sure I could go through another failure.