The RED flashing LED (which may be accompanied by a beeping signal) indicates either that the battery is low or that it is being used out of range. If the mobile units flash red 2 times every 5 seconds, it indicates that the battery is running low.

Swap the mobile units by taking the fully charged mobile unit from the docking station and placing the drained mobile on the charger instead

or

if the mobile units are flashing red every second it indicates that the mobile unit is out of range. Please move closer to the other mobile unit.