If you are lucky enough to plan a new office relocation, you probably are going to need to re-cable that office. You'll want to make sure every desk location has a connection for phone and computer.
I am here to tell you not to invest in Cat6 or Cat6a. Quality Cat5e cable and jacks will be more than adequate for 99% of users in an office environment. In addition, properly installed Cat 5e cable will support Gigabit ethernet which is more than adequate for VoIP and a shared network connection between your phone and computer. We've done this configuration for many customers with 100% success.
Why would you need Cat6 or Cat6a? Perhaps you are a financial trading
firm, engineering or architectual firm processing large amounts of data.
That may warrant a higher quality cable. Or if you own the building or
you plan to stay for many years, then it would make sense to invest in
the higher quality cable for many years of reliable use.
Many customers lease their office space. Why invest up to 50% more in Cat6 or Cat6a cable when in a few years time you may leave that space altogether? You could be throwing thousands of dollars out the window. It would be wiser to invest that money in newer LAN equipment or new computers.