It would be 20% for employees and 5% for students (?).
I am not aware of T-mobile and Verizon or similar other services.
All you have to do is enroll into their program. I do not know if your wireless carrier has some kinda agreement with your employer company. In my case it works.
Why work email? This is exact answer from their page:
We need your work/school email address to determine if you are affiliated with an organization that qualifies for additional savings. ATT has business agreements with thousands of corporations, government agencies and educational institutions to offer wireless products to their employees and families at a significant discount.
Well, I was not aware of such discounts. One of my friend is getting 20% because he claimed that he was TA and he is paying the GSEU fee for communication workers of america. I do the same, let me see how it works.
If you were not aware of such discounts through at&t, please go to
att.com/wirelessdiscounts or search for discounts. After you are verified you can get discount.