I do not receive phone from ATT, there is coax cable coming into the modem and I live in a normal house, not gated or anything else like that. I do get 12MBPS from it though.
I'm not sure why anything is going into the phone line port on the modem/gateway if you're only getting Internet and not phone service. 12MB bandwidth is too high for regular DSL service (which maxes out at 8-9mb) that can be transmitted over regular phone wire. Coax on the other hand can easily support 12MB service - so the coax connection would appear to be the physical link to the NID. An easy test is to simply try temporarily disconnecting each: if the Internet is lost with the phoneline connected but not the coax, then coax is the link - and vice-versa if Internet is lost with coax connected but no phoneline.
If the link from the NID is coax (as I suspect) you can disconnect the coax from the modem and add another length of coax to it with a barrel connector. Alternatively, if there is already coax where you want to place the modem (perhaps coax connected to a nearby TV) then you can use a ($3) coax splitter to connect both the TV and the modem.
If 'phone' wire of some sort turns out to be connecting the NID to the modem - it may be that some newer method is available to accomplish this over short distances which supports 12MB (but it would be news to me). There are in-line connectors for phone cords - you could test this option without even moving the modem. Get an RJ11 inline coupler (like below - they're easy to find) and use a regular length of phone cord (any length) to extend the phone cord going into the modem. If the Internet connection is undisturbed (you should reboot the modem) then you can use as long an extension as you need. If the Internet is lost then something about original phone wire going into the modem is not 'typical' (or least can't be extended with regular phone wire). It may be that the original installer always connects phone wire whether needed or not in case the customer ever does subscribe to phone service it'll be available immediately.
RJ11 inline coupler: