SIP over cable modem

For quite some time I’ve been wondering why I can’t make asterisk SIP calls (from softphone on my ubuntu 12.04 desktop) over my already paid for cable modem Internet service, where I currently make VOIP calls with my POTS phone that plugs into my cable modem - rather than needing to sign up for some alternative ITSP and pay yet more fees to transmit & receive voice data over the Internet (currently I use Google Voice). I can transmit and receive other kinds of data packets, so why not voice packets?

The MTA/modem has an ethernet port and plugs into my computer so that I can get Internet service, and another port for my phone to plug into. So it seems to me that the hardware connection is already there, it’s just the software that needs to be configured in some way.

I recently learned that I can supply my own modem, and my ISP/phone co. gave me a list of compatible modems. There’s about 2 dozen or so to choose from, but the first one on the list is the Arris TG852G, another random one is Cisco DPC2100, Netgear CMD31T and Ubee (Ambit) U10C022.

So my question is, if I were to buy one of these (or any on the list) or even use the one I currently have that I rent from the cable company (Arris TM602G) how do I get from there to here?

Please be aware I’m not a telephony guy so my knowledge is extremely limited. Feel free to assume I know nothing, I won’t be offended.

Thank you in advance.

This forum is for discussions, not support questions.

Most of the information for this will have to come from your ISP/ITSP.

If they have a standard SIP service underlying their phone service, the rest is stuff for which you should not need support (e.g. see the external connectivity chapter at, or should expect to pay for it.

If it is non-standard, and you are able to obtain technical details, you can come back to the Asterisk Support forum, but you should not expect a complete system design, only help on specific details.