Openstack
I recall in the earlier days of Openstack, AT&T was a big name in that space. I was surprised to see these articles around VMware and AT&T, though it made sense that AT&T used some VMware, had no idea they had so many systems on it.
See this from 2016 and 2018
https://about.att.com/innovationblog/openstack_cloud
https://about.att.com/innovationblog/airship_for_openstac
Seems Openstack was deployed at AT&T as far back as 2011
https://www.openstack.org/blog/openstack-deployments-abound-at-austin-meetup-129/
The last AT&T VMware article I poked around more and was quite surprised to see AT&T seems to have withdrawn from Openstack, maybe VMware gave them a deal too good to pass up, perhaps that is why their new bill is 1000% more expensive..
You can see here, in 2020 & 2021 AT&T was a Platinum sponsor of Openstack:
https://web.archive.org/web/20201027213028/https://www.openstack.org/community/supporting-organizations/
https://web.archive.org/web/20211019122932/https://www.openstack.org/community/supporting-organizations/
but as of 2022 they dropped off the list, I could not find any other search engine hits as to why AT&T was not on the list anymore:
https://web.archive.org/web/20220515130825/https://www.openstack.org/community/supporting-organizations/
(disclaimer I have never used openstack, I had high hopes for it when it first came out(mainly during the VMware vRAM fiasco), but by ~2014ish time frame I came to the conclusion it was too complex to be useful for anyone but orgs with large amounts of resources to support it, and could not replace simple VMware deployments, and it seems that hasn't changed, but AT&T has such resources and could do it if they wanted to).