sounds like the phone is set up for static ip addressing and is sensing the ethernet carrier detect and looking for a non existant gateway ( i.e , one that is not on your network)
Try this.
1. Obtain, beg, borrow or aquire by any means a siemens specific mains psu for the optipoint ip 410/420 phones ( this has an RJ 11 to RJ 11 lead with it)
2. Disconeect the phone from your LAN and connect to the siemens psu using the telephones psu jack underneath the phone ( the jack is marked with a relief diagram of a plug-top psu on the base of the phone)
3. After a short while the telephone will display 'network disconnected' Now press depress 1,0,3 on the dial pad at the same time and you will be able to acces the phone admin menu. (password 123456)
4. To factory default the phone = when 'network disconnect' appears on the display, depress the 1,8 and 9 digits, select factory reset using the menu arrows and punch in the following password when prompted = 124816
Hope this helps, any more problems, post again and i will try to help