Stay in touch with AGT

This is another very old mug from a company named AGT.  Unfortunately, I can't seem to find any information on the company,other than the name.  The front of the mug has the phrase "Stay in touch with AGT" along with the company logo.  

On the reverse side of the mug is a push button telephone (remember those?) with a dark area across the front. 

When hot liquid is added, the dark area turns clearer showing the name "Exten Voice Messaging." The dark area is a bit old, and not near as opaque as it should be, but it's still a great design.  I only wish I had more information on the company.  

DESIGN:  1+ Sided
THERMOCHROMIC: 1 Color
TYPE: Corporate Freebie