Mobile phone takes PCs IP addresses at random

Hi, I have 3 wireless PCs connected to my home network, all on W7 Home Premium. My Router is an Actiontec R1000H. The PCs have their own IP address and use the opendns servers. Recently my son got a mobile phone and what happens is that almost on a daily basis one of the PCs goes off-line due to an IP address conflict. This is due to the fact that when the mobile phone connects to the...