Here is what you need.
If you have Comcast Phone and want to keep it:
Put your Gateway device in Full Bridge mode and buy your own high quality router so you have control over your own network including all firmware upgrades and do not have to rely on Comcast crippled firmware.
Telephone modem only, no Gateway? Buy the Arris TM822G and save yourself the $10/month rental fee.
However call in or go to your local CC service center (the better choice) and inquire / confirm as to whether or not customer owned eMTA's (modems with phone support) are allowed to be used on the local system. Some local franchises still do not allow it and require you to rent one from CC. (EG recommends)
If you do NOT have Comcast Phone or want to get rid of Comcast Phone:
Buy you own modem like the highly acclaimed Zoom 5341J ($70 Amazon) good for the next 5+ years and save the $10/month rental fee. Buy your own high quality router so you have control over your own network including all firmware upgrades and do not have to rely on Comcast crippled firmware.
Get Ooma Phone Service (Ranked #1 Home Phone service 4 years running, Comcast not in the top 5) for a onetime cost of about $100 and no monthly fees. Now you have full control over your phone with a Community Blacklist with thousands of blocked numbers and your own 256 Personal Phone Blacklist. This will save you $450 per year alone.
Why you never want to bundle Comcast services
ever:
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Review of Comcast XFINITY by wchillman