If the modem is being leased/rented, it seems reasonable that one would call Comcast if something should go awry. After all, they still own that piece of equipment. If, on the other hand, one were to purchase their own modem from Best Buy because they don't want to pay a modem fee every month and it dies it seems highly unlikely that Comcast would service that particular piece of equipment for free; that doesn't make any kind of sense. In that case, Motorola would indeed be the logical party to contact assuming that the modem is still under warranty.
Eh, expect in one hand...