Synergy 3505 twin phone?

The phone was working fine, but all of a sudden it now rings only once, then bleeps and as you try to answer it nothing happens so it just carries on trying to ring. the base displays base 1 and shows 4 dash's on the screen, it seems the battery is flat, when i place it on the base and the battery is fully charged and all seem ok until it rings again then i can't answer it again.
I can ring out but it cuts off dead after about 2 minutes with no warning.
Please help
Thanks