If it is a "wireless" connection between computer and router which is being lost when using a cordless phone, you could try changing the Channel number used by the router to a high number, such as 11. Cordless (and mobile) phones are one of the more common causes of dropped WiFi connections and should generally be kept away from Routers and Wireless Network Adapters.
But if you're wired only, the most likely problem is a missing filter (as per jollyjohn) or an existing one is faulty. Try unplugging everything from the phone system except the router and one phone. See if connection drops when making a call. If it does, change the filter. If it doesn't, add other units to the phone line, one at a time, until you discover which filter is causing the problem.