This article explains what to do if the signal strength LED is flashing red.
INSTRUCTIONS & INFO
Unplug the WHP Base Unit, fold the antenna down, then turn the device over.
Gently press down on the raised arrow on the back-up battery cover, then slide it in the direction the arrow is pointing.
Pull the three-prong connector out of its port and remove the back-up battery.
Take the SIM card and insert it into the SIM slot, with the notch in the upper left corner and the gold contacts facing down. Slide the SIM card in until it clicks into place.
If the SIM card is missing, or present but the signal strength LED still flashes red, contact AT&T Customer Care to obtain a replacement SIM card.