I don't think companies pay for you to go do an MS or a PhD full-time on company dime. At that time AT&T was a monopoly and may have had money to burn on this. There also may not have been expectations of hyper-growth from the stock market that exist today.

AT&T still pays for various MS courses (mostly MSCS, MS data science and cybersecurity) you can do on a part-time basis. It's quite easy to get the tuition reimbursement for it.

In general, nobody needs to pay someone to do a PhD, at least in a scientific or engineering field -- even in the US, the country where education generally costs the most -- typically PhD students get free tuition as well as a living stipend. Masters students, granted typically don't get this benefit -- but in the US that's typically a terminal degree -- unlike in many countries you don't do a Masters and then a PhD -- you do one or the other.