I used to have an "Unlimited Data Plan", which a customer service rep adeptly convinced me to change to a lower data plan to "Save Money". Interestingly, I found after I changed plans, my phone got faster. I never put two and two together. (It didn't save much money either, because they then charged a Smartphone fee).
They were "Throttling" my data to give me slower data speeds. AT&T claims that they sent emails and text messages to customers that once they pass a certain amount of data, their speed was turned down.
I never got one email or text saying such.
As soon as my contract is over, good bye AT&T forever.
[Edited to comply with Guidelines]