You can call, but the most effective way is to carry Comcast's equipment to the nearest local store and turn it back in to them. Be sure to get a receipt, and keep it forever. Also consider scanning or copying it; my shop uses thermal paper, which fades to a blank piece of paper in a week or so. Bear in mind you may be under a contract and thus subject to an early termination fee. The official word on early termination fees: http://forums.xfinity.com/t5/Billing/Early-Termination-Fees-ETF/m-p/2895117#M96760
To close your account:
Drop off your equipment at any Comcast Service Center or XFINITY Store. Close your account and retain the receipt. Find a Comcast Service Center or XFINITY Store Location near you.
Comcast has teamed up with The UPS Store locations across the country to accept Comcast equipment returns. Simply bring your equipment with you to any The UPS Store location, and an associate will pack and ship your equipment back to Comcast. Visit www.theupsstore.com to find The UPS Store locations near you. The UPS store will give you a receipt with a tracking number so that you can track the progress of your return on UPS.com. Please allow up to two weeks for the return to be processed on your account. Call Comcast 1-800-COMCAST (1-800-266-2278) or 1-800-XFINITY (1-800-934-6489) and close your account.