CISCO IP PHONE SOLUTION?

in a small office, they got 4 og lines connected to a pbx and spreaded between 20 employees....despite the fact whatever system they are using, i will appreciate if someone can help me to setup cisco ip phone in their office, with full functionality...i also need some help to purchase the right components.....how many routers/switch, ip phone (i guess its 20), computers, any other components i should consider...