You didn't give us any idea what the problem is, but some random thoughts on returning equipment: 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.
Maybe that will help. If not, please provide a little detail.