I have a Nokia 920. Updates for the phone comes from Nokia, not AT&T.

Check with HTC.

If I recall, the last update released by Microsoft for this phone was Mango which was released by AT&T in October of 2011. 

Can't really tell you here so try to ask around on the forums of wpcentral. You'll find out how because even though it might not be on att servers you can get the cabs to run it. Also ask about debranding it from at&t

Microsoft provides the updates and the Carrier ATT holds the updates back the last update for my device was MANGO and TANGO came out months ago. ATT promised it would be available by the end of the summer but I still don't have tango which is why i can't flash 7.8 to my  phone since the update follows in order. The cab file method would leave me stranded on Tango. HTC and ATT both lack support for old customers and even though there are Nokia phones on ATT I much perfer T-mobile for billing reasons. My phone is unbranded i actually switched to t-mobile a while ago but the updates are still managed by ATT. I only ask that an ATT member reading this post can please pass on the word that the windows phone 7.8 update is important to all windows phone users.

Can you please mention this to a department that handles phones and update proccess because microsoft says the carrier handles the updates and your saying that the last update Microsoft put out for you guys to release is Mango. I just really want the updates on my phone one fixes a bug where your keyboard disapears, once gives you internet sharing, and the other gives you a very nice start screen that I dream of trying out on my windows phone.

http://www.wpcentral.com/htc-abandoning-hd7-no-plans-release-windows-phone-78-update

That statement was proven false since the HD7 got the update to 7.8 on another carrier in Europe. 

Well maybe you should do a little research. Microsoft released a couple major updates since October 2011, however AT&T has continued to sit on them. 

I have an AT&T Samsung Focus in front of me that is also lacking the Tango update from last year and a still on contract Focus S that recieved the Tango update 3 months after Microsoft released it. Removing the AT&T carrier lock allows the phones to be updated as soon as Microsoft releases it, otherwise we have to wait until someone at AT&T decides to approve the update. 

Given all the wonderful service I've recieved from AT&T over the last 2 years as a customer using Windows Phone, I can't wait for my contract to be up so I can take my business elsewhere ! 

Zune notified me that updates were available last night and my (Unlocked HD7 though with ATT service) updated to Windows 7.8 (7.10.8862.144)

So Far looking good