Specifications:

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Size: 2"x1 1/4"

Connection: Two RJ 11 telephone line cords jack.

Color: Ash

Model: Tes-856LP
Applications: Centrex/Residential, IP also some PBX and Key telephone systems.
Signal required: Message Waiting Visual Feature must be turned on from your voice mail provider.

Model: Tes-326

Application: 90V. PBX and Avaya PBX key system. 

Signal required: 90V or Avaya signaling.

Display their FAQs

Will the model Tes-856LP or Tes-326 work with stutter dial tone.-NO, Most telephone system turn on the message waiting light using 90V or Avaya signal for the Model-Tes-326. And for the Tes-856LP the CLASS/FSK signal is used. It is known as a feature called Message Waiting Visual. You must have this feature on for the light to work. Most of the time it is turned on when they set up the voice mail box.