AT&T blocking our IPs - need assistance

The ISP I work for has a large block of IPs (randomly distributed) being
blocked by AT&T so that mutual customers cannot log into online portals
like and The common nodes of failure are and

We have had no luck getting through to higher tiers from customer-facing
phone representatives, nor have we had a response when leaving a voicemail
at 314-235-8168 (generic WHOIS registrant phone number, but recording
mentions "domain administrator"; please check your voicemail and response;
we just left another).

Please have an AT&T server administrator reply off-list in regards to
fixing this.

Thank you