There have been posts on buying a Cable modem like the Motorola SB6120 or SB5101 to avoid the $7/mo Comcast modem rental charge. However, if you use the COMCAST Triple Play, these don't work since there is no landline phone line output from these modems, just a ethernet output for a computer.
So is the only option to buy the modem from Comcast like the ARRIS TM602G? Even buying a used Comcast modem from a third party risk duplicate MAC addresses which must be resolved by COMCAST to provision for it.
As far as I'm aware, the only modem you can buy with phone service is the Arris TM722G, only from BestBuy with a Comcast Connection Center. Only if this is permitted in your market and will require a paid tech visit to install it.