You obviously didn't try Google!

Search for the whole phrase ... they had their own building! It was one of the many small phone companies that started up and were merged into the ATT monster. A history of ATT might help you.

Ant there's a museum that might like to get the phone, especially if your great-grandparents were swedish

I can't help you with your phone but here is a funny story.

A friend was working last summer on building his house in rural Minnesota. Right next to the Iowa border. He needed to call a plumber in the next town. So he called using the same area code since they were only eight miles apart.
He got a recording that said:
"Uff da! You didn't use the right area code, don't cha know!" The name of the phone company was The Norsk Phone Company.

It's likely that this phone was manufactured by the Ericsson Co. of Sweden, which formed a partnership with Alexander Graham Bell.
Here's some history I found (excerpted):

"Lars Magnus Ericsson began his association with telephones in his youth as an instrument maker. In 1876 he opened his own company, and by 1878 he began producing his own telephone equipment. His phones were not technically innovative, but because his co. maintained equipment for Telegrafverket and the Swedish Railways, he was familiar with the telephones of the Bell group and the Siemens Halske Phone Co. He was able to improve these telephone equipment designs and thus produced a higher quality instrument. Ericsson's training as an instrument maker is reflected in the high standard of finish and the ornate design which makes Ericsson phones of this period so attractive to collectors.

With Ericsson's reputation established, the Ericsson Co. became a major supplier of telephone equipment to Scandinavia. Because the factory could not keep up with the demand, work such as joinery and metal-plating was contracted out. Much of the raw materials was imported, so in the following decades Ericssons bought into a number of firms to ensure supplies of essentials like brass, wire, ebonite and magnet steel. It is interesting to note that much of the walnut used for telephone cabinets was imported from the USA."

My DH is an employee of AT&T, so this topic is of interest to me. By the way, Ericsson is a major supplier of electronic, computerized switching equipment that AT&T widely uses even today.

I also have a wall Swedish American Telephone. Mine was reconditioned by western electric in April of 1907 I came into possession in 1968.