You will need either a Voice Modem OR a telephony board.
Voice Modem: Relatively cheap (about $35 USD). Acceptable audio quality, although not the best. Great for home installations, test set-ups & home-offices. Not highly recommended for large businesses. Perfect for set-ups where you want IVM to answer the call (either with an IVR or Voice Mail after a certain number of rings) without doing much else.
Telephony Card: Quite expensive ($200 USD and up). Very high audio quality. Typically supports more than one line (voice modem usually only does 1). Great for business or home-office set-ups.
Can be used in conjunction with NCH's free Carousel software to convert the lines to VoIP. You can then set-up your own internal VoIP network and have several extensions which can call each-other or make outbound calls. You can also have complete control over call routing so that for example when someone phones in you could set it to ring extension x for 4 seconds and then ring extension x, y and z for 15 seconds (just as an example). There is an endless limit of possibilities if you choose to set-up a VoIP network, if you have any other questions please do ask. You do not need a good Internet connection for an internal VoIP system, just a standard LAN.
1) Here is a page on NCH's website with very detailed information on Voice Modems including a compatibility list for IVM
You can also buy Telephony Cards direct from NCH here.
2) Not sure where you're located, but here are some places to try:
USA: Best Buy, Circuit City, Radio Shack
CANADA: TigerDirect, Future Shop/Best Buy, The Source
AUSTRALIA: MSY Technology, Dick Smith's Electronics