During the ESD testing of a Dual T1/E1 line interface unit (LIU) at Level One, the circuitry in every pad cell was found to be robust to Electro-Static Discharge (ESD) threshold of up to 2000 Volts. Device failure was however observed on devices when functional tests were performed. These failures were observed on devices which were only subjected for 1250 Volts of ESD stress. The failure analysis revealed ESD induced damage at the Gate - Drain region of a MOSFET in the core of the die. A weak power supply clamp in the supply circuit caused this failure. The step by step analysis process and the redesign of this device with confirmation on new design revision is explained.