I would use one of the cat 5's for the network and you said that the customer has a phone system, depending on haw many pairs the system uses to connect to the phone you could be short a pair or 2. If it is just normal POTS lines you could terminate one pair onto one module to give you capability to have 4 phone lines and 1 ethernet line. For a home or small office like that I like to run 4 cat 5's, 2 for voice and 2 for data, which gives you more flexibility.
