If I had to guess, I'd say you have an 802.11g or 802.11b network and a 2.4GHz cordless phone, which is the problem: both your router and your phone operate in the same 2.4GHz frequency, and they're competing with each other. This is exacerbated by the fact that they're in the same room. Interference from wireless products in the 2.4GHz frequency is a known issue for 802.11g/b networks; even baby monitors and microwave ovens have been known to wreak havoc on Wi-Fi networks.

You can try a number of things for free, or you can pony up for a new phone. First off, try moving the phone's base unit into a different room from your router/access point. Also, try switching channels on router/access point. If these suggestions don't work, you can invest in a cordless phone that uses a different frequency than your wireless network: 900MHz or 5.8GHz.