Blocked by ATT

Hello, we are a small IT company that has the same static IP address for many years (over 10).  Our ip address is 198.0.112.205.  We don't send spam, and we have proper spf/dmarc/dkim records (have for years). Several years back we realized that we were unable to email anybody with a sbcglobal.net address. We get the "DNSBL:RBL 521< 198.0.112.205 >_is_blocked.For assistance forward this error to abuse_rbl@abuse-att.net"  We have forwarded the error to abuse at att, multiple times.  One time out of the 20 or so emails we did get a reply of "24-48 hours should be fixed"...but other than that ...*crickets*.  Can anybody help us?