Are you saying you have a phone and when you are on the internet (I assume a dial up connection) that when your phone rings you lose your connection to the internet? There should be a setting under your modem properties to to set so this doesn't happen. You more than likely have call waiting as a feature for your phone and this is why you get diconnected.
The cordless telephone and the wireless router may be using the same freq range (2.4GHz). You may need to get a corded phone or one with the 900MHz freq and the latest 5.8GHz..