Cisco WAN :: 877w Device Disconnected Automatically

A client of mine is using a Cisco 877w wireless router, they have one IP phone connected to one of the RJ45 ports on the back of the router and their main desktop is also plugged directly into the router - this all functions perfectly.However, the router is in a private house and as such it provides connectivity for all other wireless devices such as iPhones, Ipads and Xbox Live etc. The problem is that the router only allows 5 devices to be connected at any one time and when a sixth attempts to connect to the router the first device (the IP phone) is diconnected automatically.
 
So my client can be using the IP phone for an important call and his son walks in with Iphone and Xbox, friends etc etc and the IP phone is immediatley disonnected from the router. I beleive that the router should allow more than 5 devices to connect and would hope there is a way of stopping the IP phone constantly being disconnected.I have already paid for "Cisco Engineers" to attend this site but they cannot get it working.