n00dy: Although everyone here seems to be pointing to Voip, The LG system uses your existing lines without any tech know how, if you want an easy to setup, easy to use and inexpensive (less then $2k) the LG SoHo. Plugs into existing phone jacks, runs corded and cordless extentions, allows keyphones just like to big expensive PABX, allows analogue extention for Fax, eftpos or Broadband
http://www.atlasgentech.co.nz/products/index_dynamic/product/5924
OP said doesn't want to have the extra monthly's of a extra pots line