ARP is only for the 5100a as it is a Bridge without an IP until you give it access. 5100b doesn't have Telnet enabled after it is configured. I understand it may work after a reset and before it is set up/configured before it is plugged in, but once it hits DSL signal and sees sync, Telnet is disabled (don't recall exact steps as this was 4-5 Years ago).
Your best bet is to work with AT&T/SBC Direct here »AT&T Direct . There's nothing you can do to the Modem to make you line stable (except move closer to the DSLAM).
