Failed daily call

One of my two Tivo's has suddenly started to get 'service unavailable' when it attempts a daily call. I can tivoweb to it and telnet via putty, so it's coonected to the router ok. Checked the network setting, all seem ok, except the subnet mask on the faulty machine is 250.250.250.1 and on the good machine it's 255.255.255.1 I tried to rest it to the 255 settings via nic_config_tivo, but after retyping the (ststic) IP address, it skips straight to the gateway line. any ideas? This machine has been flawless for months!