Why does the EXT-401 Explosion Proof Telephone require an external signal?

The nature of the EXT-401 explosion proof enclosure with its thick, sealed copper free aluminum walls makes it impractical to effectively transmit sound from the inside of the enclosure.

Guardian Telecom offers a variety of options including the CE20 line-powered ringer, configurations of the SR40 Speaker/Tone Ringer and hazardous area strobes.