Modem signals are very tricky and sensitive to wiring problems. In my house There are six phone jacks in different rooms, all on the same line. Most of the time I can connect at 44k max, but if unplug the phone on one specific room, I can connect at 52k. It's not the phone itself, just when any phone is plugged in that room. Unfortunately is one place where it has a lot of use, and can't be permanently unplugged.
