This type of problem can happen if there is any abnormal power issues or power outages. Its also common to see issues if the ATA is backfed into the homes internal wiring which is why we do not recommend that type of setup.
What do you mean by backfed? I have an HT802 and have experienced unexpected disconnects and device not registered.
We used to have a Leviton phone patching expansion board (47609-EMP) connected to the ATA so I could use 2 phones. Sonic support recommended we don't use it. Having just one phone with 5 handsets connected, the disconnects have subsided quite a bit but we still get device not registered on occasion. The last was probably 2-3 months ago. The ATA is connected to an old reliable pure sine wave APC UPS. The second phone had its own power adapter and has REN 0.4A, ringer equivalence of 0.7B. The simple Uniden phone provided by Sonic has REN 0.7A, ringer equivalence 1.0B.
An issue with the HT802, and probably HT801 is if there is a power outage or if my modem reboots, the ATA doesn't retry authentication after the modem comes back on line. One has to reboot the ATA via ***999 or power cycle manually. I know it doesn't do it automatically because the last time ATT upgraded my modem a month ago I forgot about it, went to sleep, and when I got up the ATA still wasn't registered.