How can I connect 3 offices with cheaper cost

Hi all
How can i connect 3 offices using phonelines? and how can i connect like an intranet or a wan? What kind of hardware i should use and setup? I heard about DSLWICs (DSL WAN Interface card). Is it possible to connect between 3 offices far 7km? Here in Mongolia uses WINDOWS XP and Lunix(Often Windows). If you all have any idea, please help me… 🙂 I would like to learn about technical connectivity. 🙂
Thanks all