All or at least most IP phones these days operate via 100BASE-TX protocol, which is designed to run on CAT5. The CAT5E spec was created to support 1000BASE-T, so at a first glance it may look a bit redundant to use a better cable. However, in a new installation today it would make much more sense to install CAT5E instead: the price of the cable is the same (if you can even find CAT5 cable), but the quality is surely better. The way things are on the market today, people tend to call CAT5 every data cable that's of poor quality or unknown origin, or simply old. Therefore, I would suggest that you go for CAT5E and enjoy all benefits of the superior technology for the same money.
Dmitri Abaimov, RCDD