7 year old bill sent to collections

You have to deal with the collection agency. 

How is ATT still sending different agencies a made up bill after 7 years if they cant pull it up to tell me what it is? 

I called the agency they just basically fought me and said it is what it is. ATT says this and this is what you have to pay, and I'm not paying for anything I don't actually owe. I also disputed the letter before the date on the letter but they added it to my credit report anyways. 

Companies "sell" debts to collection agencies for pennies on the dollar.  Whether you owe the bill or not, let's break this down:

AT&T has on their books a debt that they claim you owe.  Let's say $500.  They try to collect it for a period of time.  Eventually, they 'sell' it to a ABC Collection agency for $100 just to clear it off the books.  AT&T gets something and the debt now belongs to ABC Collection Agency. 

The collection agency's only job is to try to get the debtor to pay, hopefully the full amount BUT they are usually willing to take something less.  If a consumer admits that they owe the money, they can get the balance to $0 by paying a negotiated amount.  Unfortunately, this does not remove it from their credit report but it does show a $0 balance.

If the consumer chooses to dispute this debt, there are steps to be taken.  Check out the information shared by one of the 3 credit reporting agencies, TRANSUNION

No one here can tell you why this happened but at least now you have some working knowledge of how the transaction came to be.

That's already been explained. AT&T sold your debt they no longer own it the collection company they sold it to, or the one that that collection agency again sold it to now owns the debt.

FYI... T-Mobile (or any switch offer) pays you so that you can pay off your previous provider. No cellular company directly pays your old phone company off. They can't. So if you are on to the impression that 7 years ago T-Mobile paid off AT&T after you left, they did not.  They could not. 

   If T-Mobile didn't pay you enough money to pay AT&T off, or if you just never paid AT&T off in any way, the debt is yours and you are rightly being pursued to pay off something that you did not pay.

This is true no matter which service provider offers a switch offer or offers to pay off your old phones. They cannot access your account with your previous provider.  Privacy laws forbid someone else from accessing your account to pay it off.   

How is ATT still sending different agencies a made up bill after 7 years

They're not. The collection agencies sold it to another agency. But I agree with @formerlyknownas that based on your post stating T-Mobile paid AT&T, it's probably a legit debt. 

7 years later, after switching to T Mobile and they paid for the switch, which is why I have the same phone number. 

It sounds like you might be confused if what @formerlyknownas is correct (which I don't doubt it).  Hopefully you have some sort of documentation from T-Mobile otherwise it is a valid debt that was sent to collections.

Yeah....sounds like T-Mobile failed to pay for the switch which means the debt went into default.

Which no carrier does to my knowledge. In any switch offer, the new carrier credits or reimburses the new customer, they don’t pay the old carrier. It’s still the customer’s responsibility to pay the old carrier.

T-Mobile's offer 7 years ago would have been similar to their offer today which refers to the payoff as a "rebate". 

https://www.t-mobile.com/offers/switch-to-t-mobile-from-verizon-or-att