Cat 5 Wiring Existing – Shared with PBX

I am attempting to install a network at a business in which the cat 5 wiring exists to each office ext. However the phone system runs on the blue and blue/white wires. Currently the fax/data line runs on the orange and orange/white wires. The business owner is positive that the phone co. told him it will work to network the remaining wires. Is this true? Does a network only use two pairs. Or do I have to take over all the wires. Thanks for your help.