AT&T Threatening Overages Without Providing Meters? → Just One Email

When I got the email notifying me I had exceeded my "Data Plan" for the month I immediately began a search for an alternative internet provider and within a couple of days signed up with DSLExtreme.
I wish I could also drop the landline just in spite.

Why can't you?

Can't drop the POTS line and use a 3rd party ISP with AT&T.
They are still getting my money indirectly thru DSLExtreme but there are no caps...for now.

Actually, I did just that. Requires a CLEC with their own POTS gear in the CO. I don't believe DSLExtreme does that in any state that they serve, but a small, Northern California company (Sonic.net, LLC) does; mostly in the S.F. Bay Area.

Because DSL requires a landline and in most areas in the US, AT&T is the only landline phone service available.

For some values of, "most", maybe. But AT&T only covers 22 states; and not all in most of them.

I didn't wait for the "letter". As soon as my meter was active I dumped AT&T for Sonic.net "Fusion". I got twice the download for half the price, and "Fusion" includes POTS. Ported the number over. I am somewhat miffed at a minor change with Sonic.net, in that I will be leaving the old account with the homeowner, so my status will be, "new customer", and I will be stuck with a modem rental fee. But I hear that AT&T IPDSLAM service is the same way.