I do not know the details of how VOIP works, so I can't be sure I am right, but I always write like I know what I am talking about, so....

Let's back up one step. Or more.

Was your sbc phone line connected to your receiver in any way? for that matter, I am just assuming you are talking about an audio/video receiver, right? What brand and model (it might matter)?

Comcast is a cable company, right? That means they connected something through the cable...which is grounded. And that ground connects to the ground of It should be grounded to a power ground as near to where the cable comes into your house as possible. This does not always happen, though.

But was there any connection between your receiver and the sbc lines that is now different from what it was before? For that matter, is your receiver connected to a TV or a cable box that is connected to Comcast for cable?

Sorry, but in the course of working this out, all of this needs to be known and most preferably shared here in this thread as you have asked a question that is really a bit too vague for any answers that might help.

Meanwhile, try this: disconnect any and all cables from Comcast that were not already connected to something inside your house before VOIP, and see if your AM reception comes back. If it does, then Comcast needs to work out your solution.

Either way, please write back and expand on your question. This could affect others of us, and details will be helpful for all.