There are four Comcast approved retail devices that handle voice:
The all-in-one Arris SVG2482AC
This device handles voice, Internet, routing and Wi-Fi. This all-in-one type of device is commonly referred to as a gateway. The SVG2482AC is the highest performer of the group with 24x8 channels (down/up). This device can be activated across Comcast's entire footprint.
The all-in-one Arris TG862G
This device handles voice, Internet, routing and Wi-Fi. This device can be activated across Comcast's entire footprint.
Those that may need better router/ Wi-Fi performance may wish to use their own router. You must disable the router/ Wi-Fi capabilities of the TG862G before you can use your own router. This is known as putting the gateway into bridge mode. Bridge mode can be accomplished by following the directions found here.
The Arris TM822G
This device handles voice and internet. It does not have a built-in router.
The Arris TM722G
The TM722G is an older device that handles voice and Internet only. This device is notoriously difficult to find at retail, be very cautious if you choose to buy a used one as they are often stolen equipment and can't be provisioned on Comcast's system. Also be aware that the TM722G is an older device (4x4 channels) and will not support Comcast's faster speed tiers. I strongly suggest that you consider a newer device; the TM722G will likely be obsolete before it becomes cost effective. Call 1-800-COMCAST (1-800-266-2278) or 1-800-XFINITY (1-800-934-6489) before purchasing a TM722G to be sure Comcast will allow you to use it in your area.
Before you start, make sure your account number and telephone number are readily available. This information can be found on your monthly statement or order confirmation e-mail.
If the online activation does not succeed, call customer support: 1-800-COMCAST (1-800-266-2278) or 1-800-XFINITY (1-800-934-6489)
[Updated 4/25/17 by nerdburg]