The local phone company should be able to provide you with enough buried drop wire that you can place in conduit for free. I work for Frontier Communications and deal with this stuff all the time. Do not put in a Cat 5 exposed or in conduit, unless it is specific to outdoor use. The wire used indoors will typically last a few years, then people are calling me to figure out why their phones or DSL are not working. If you are going to be placing the conduit, and it's not too much out of your budget, place 2" for future use. The major phone companies are starting to use bonded copper pairs for faster internet service, and in the future 4 or 6 conductor wire may not be enough. Pull strings only work for short runs when other wires are already in place, otherwise you use it to pull both wires at once. If a new wire needs placed, replace all existing wire at the same time, using old wire as your pull. The least amount of bends the better.
Edit: Another reason for using the proper wire rated for buried service is the shielding inside the wire that is bonded to ground to prevent cross talk or power influence.