3784kbps on Comcast cable.
One of the things that may be an issue is cross-pollination. I know there's a more technical word for it (like interference), but if your phone lines are running parallel to electrical wires and are within 8 inches, the electrical wires will generate interference and slow down your speeds. You might take a look for that... I did the computer wiring in the basement home office here, and I did it twice to make sure I got it right
'Cause I never make mistakes, oh, no sir...
BTW, is there also a filter right at the phone box in the house? 'Cause there should be, IIRC.