If you're getting your home phone through comcast you'll need to buy/rent a modem that has the voice line in it. As an example from their website, the Netgear C7100V. From there you plug in your normal landline phone.
Alternatively you can get an VoIP phone setup like USAFRet suggested (Ooma, MagicJack, Vonage, etc). They're typically a lot cheaper than phone service through your ISP.