That depends on how the phone is connected. I'm assuming it's a VoIP phone. If the phone is connected to the computer, which is then connected to the internet connection, then the computer must be online for it to work (the phone is getting its connection THROUGH the computer, so if it's not online, then there's no connection for the phone to get), however, if the phone is connected directly to the internet connection via router or switch or whatever, then you should be able to use the phone no problem, even with the computer offline.