All the batteries have been checked. I can hear line noise (clicks/pops or faint static, no trace of a dial tone) while plugging and unplugging the phone cradle from the socket. All handsets died simultaneously, and worked perfectly before. All show 'fully charged' and 'check phone line.'
If there was something wrong with the handset and I dialled my landline number, wouldn't I get an 'engaged' tone, or have the number ring forever, rather than being instantly disconnected?
edit: if it would be helpful, I can provide account details here or in a private message.