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Did Comcast supply this EMTA or did you buy it through Craig's list or some other online source?
If it is branded by Comcast (has their logo on it) and you bought it online, it may be reported stolen.
If a friend gave it to you, they should be able to contact Comcast and clear it up. If they gave it to you, they are probably still paying the rental fee on it and probably would appreciate the chance to remove the charge from their bill.
If Comcast was renting the EMTA to another customer, they're required to return it to Comcast or pay a fee. I don't think Comcast's systems are set up to allow customers to transfer modems between each other. You basically bought stolen property. Since it's Comcast's modem, they expect to continue to receive a rental fee for it.
As for why it works on the Internet, I guess they have separate provisioning for Internet and voice, and only the voice part checks for this.
So, what can you do?
1. Get in touch with the person you bought it from and return it to them and get your money back.
2. Go to your nearest Comcast office and turn it in and rent one from them. They will be happy to get their stolen property back. Maybe you can use that to your advantage with a negotiated discount.
3. Buy a new one from a legitimate source (not that Craig's List isn't legitimate) like Best Buy, Amazon, etc. Here is a link to the Comcast-approved modems.
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