Can you use your run of cat5 and used the unused pairs for telephone?

Depends on the system you have setup with the old Cat 5 you could of as it only used two pairs I believe it was blue and orange pairs on a 586A connection ( normal RJ45 not crossover ). With Cat 5e now which has replace Cat 5 it uses all 4 pairs to run 100meg connections and up to 1 gig if you have the right equipment. If your just going to use Cat 5e for telephone and data link you won't be able to.

I have personally done this, and it works.

You only require all 8 wires for Gigabit LAN, otherwise its only the green and orange pairs used for 100meg.

I would only be splitting pairs out of cable runs as a last resort on temporary installs, otherwise run two cables at the same time. i've had some fairly crap experiences on offshore rigs where the last guy out got lazy and split pairs out along a cable run, which made it painfull to trouble shoot devices which weren't picking up the POE from a switchport.

I read some comment on running cat 6 and above for future proofing, you need to think about the cost implications of doing this. Remembering that to get the benefits the end devices need to use that bandwidth, and end devices are fairly expensive to upgrade and therefor done infrequently. I recently cabled my house and had the supplier trying to sell me cat 6 as opposed to cat 5e siting future proofing as the reason, my end devices will only go to 100mb which is plenty for a home, so not worth the extra 40$ per 100m. So try and keep it real when making the decision to purchase