Just to expand;
The current consensus is that the only customer-owned telephony modem approved by Comcast is the Arris TM722G, which you must purchase from your local Best Buy (if available) that has a Comcast Communications Center/kiosk. About $150 for the modem plus professional installation. Some stores/areas will allow a "self install". Those may supply a kit that looks like this;
»blog.trebacz.com/2012/01/comcast ··· ris.html
Also be advised that some market areas do not allow customer owned eMTA's at all. It's best to call in a see what your market area's policies are.
»www.arrisi.com/consumer/_docs/AR ··· _Buy.pdf