Since most routers operate at 2.4GHz there shouldn't be a problem. It's always advisable to connect your cordless phone a little way away from your computer equipment, though, as some interference can occur otherwise.
I have a Dell desktop PC set up in a back bedroom which is connected to a DSL modem and a wireless router is also nearby which allows me to use a Dell notebook wirelessly throughout the house. There is no phone in the bedroom right now but I'd like to install a 5.8GHz cordless phone in the bedroom and leave it. If I did install a 5.8GHz cordless phone in the bedroom where the computer is located will that 5.8GHz cordless phone cause any interference with the wireless router?