VTech DS6722-3
Registering the handsets
I noticed that after a power outage, my VTech wireless phones at home would lose all of their connections to the base. What may be happening is that the key stored on the base for the connection is being lost during the power outage. However, the message that appears on the phones can be misleading: "Out of range or no pwr at base" There are basically two steps to restore the connection to the base, and they are de-registering all of the handsets at the base, and then registering the handsets. I have DS6722-3, which was exclusively available at Costco locations in 2016.
Deregistering all handsets
At the base, hold on to the FIND HANDSET button for 15 seconds or longer, or until the "IN USE" light starts flashing. Then press the FIND HANDSET button again.
Register the handset
Press *7890# and then get to REG CLR if the message about power is still there. Hold on to the FIND HANDSET button for about 4 seconds to have a IN USE light turn on solid. Now, press the # button on the handset. Voila!