AT&T (T 1.65%) was once considered a stable blue-chip stock, but it's lost about 40% of its value over the past five years. The telecom giant's steep decline can be attributed to its debt-fueled acquisitions of DirecTV and Time Warner, the slow death of its linear pay TV business, and its costly attempts to challenge Netflix and Disney in the streaming video market.

AT&T has been trying to rectify those mistakes by selling a major stake in DirecTV, spinning off Time Warner's assets into a new company through a merger with Discovery (DISCA) (DISCK), and divesting its non-core businesses to reduce its long-term debt. Those decisions represent baby steps in the right direction, but it could take years for AT&T to become a reliable long-term investment again.

An AT&T employee at a retail store.

Image source: AT&T.

As AT&T treads water, investors should look beyond its main turnaround story and review some lesser-known facts about its business. Here are three things about AT&T that only the smartest investors will likely know.

1. Today's AT&T isn't the original AT&T

Casual investors might assume AT&T is still the same company that was co-founded by Alexander Graham Bell back in 1885. However, the U.S. Department of Justice actually forced AT&T to break itself up into smaller regional companies with an antirust lawsuit in the early 1980s.

Southwestern Bell, which was based in Dallas, was one of those "Baby Bells". Another notable Baby Bell was Bell Atlantic, which was originally based in Philadelphia. These two companies expanded via big acquisitions and mergers over the following two decades.

Southwestern Bell (SBC) finally acquired the remnants of the original AT&T in 2005, inherited its brand, and emerged as the "new" AT&T. Therefore. today's AT&T has only technically existed since 1984, and all of its financial data before 2005 actually comes from SBC's old reports. Bell Atlantic also transformed into Verizon (VZ 1.40%) after its merger with GTE in 2000.

2. It's becoming an underdog in the wireless market

For many years, Verizon and AT&T were the two largest wireless carriers in America. However, AT&T actually slipped to third place last year after T-Mobile (TMUS 0.32%) closed its merger with Sprint.

T-Mobile's low-band 5G networks also provide more geographic coverage than Verizon and AT&T's high-band networks. Back in May, T-Mobile claimed its 5G network offered about 33% more geographic coverage than Verizon and AT&T's combined 5G networks. In July, T-Mobile claimed its 5G network covered 92% of all interstate highways miles across America, compared to 68% for AT&T and 51% for Verizon.

T-Mobile's rapid growth likely set off alarm bells at AT&T, which hastily abandoned its plans to build a massive media business to focus on aggressively expanding its wireless business and 5G networks. However, it could already be too late for AT&T to shake off its new reputation as the wireless underdog.

3. Its insiders are buying more shares

AT&T's stock looks dirt cheap at seven times forward earnings, but the bears will argue that its previous blunders easily justify that steep discount.

However, AT&T's insiders have also been buying more shares this year. Over the past 12 months, they bought 1.19 million shares while selling 1.03 million shares. But over the past three months, its insiders bought 169,022 shares while only selling 80,280 shares. By comparison, Verizon's insiders didn't buy a single share over the past 12 months.

That warmer insider sentiment suggests AT&T's stock might be deeply undervalued if we factor in its upcoming spin-off of WarnerMedia. AT&T's existing investors will gain shares of the new company next year, but AT&T will also reduce its own dividend to account for that divestment.

Is AT&T worth buying right now?

I own some shares of AT&T, but I'm not confident enough in its spin-off plans to buy more shares right now. However, I also think the stock is too cheap to sell, and has more upside than downside potential at these levels.

If you already own AT&T, you might as well stick with the stock and see how the WarnerMedia divestment plays out. But if you don't, you should probably buy other dividend-paying tech stocks instead of this struggling telco.