Besides the basic functions as a common Tel Line Tester, it also has functions of high voltage protection and polarity indicatio Main Functions

Common telephone function: Dial, Ring, Talk

Mute

T/P switch

High voltage protection (by fuse)

Polarity indication by LED 

Volume adjust

Pause

Store phone number

Monitoring function

Last number redial 

Telecom Line Identifying (Telephone line, ISDN line, ADSL line)

Key Features 

Dumbbell shape, small size, simple operation

Special Dumbbell shape design

Small size

Easy operation

Solid new materials for shell

Waterproof and vibration proof

ST230D telephone line tester