Incorrect DNS response on ATT Wireless (no errors on wifi or on other mobile networks)

Have a mail server that, periodically, gets results in a certificate error on ATT-connected iPhones (only). The cert shown belongs to securelogin.hpe.com which, clearly, I don't have the key for (and Digicert didn't issue that to me). The only thing I can think of is that the DNS servers on ATT are responding with the wrong IP at some points in time. I've confirmed these phones don't have profiles or VPNs configured, and they aren't connected to any wifi. I saw one similar forum post about something similar (and the same hpe certificate), but it was unresolved.

My next troubleshooting step is to get one of these phones and tether and use dig/nslookup to see if I can get a response that is incorrect while on that network/using the phone's APN. I was unable to query the DNS server addresses that supposedly are the ones that ATT Wireless use (that showed up in another forum post, so unconfirmed - I can't confirm as iOS doesn't show DNS server address for mobile networks).

At this point I am nearly certain this is a periodic improper response from ATT DNS servers. I'd certainly like to raise a ticket with ATT about this issue, but I'm not sure of the right avenue to do that. Has anyone had a similar problem they have solved? Or a contact to raise the issue?

Thanks!