Spamhaus PBL block on dynamic IP

We recently dropped our static IP.  Our self-hosted PBX cannot email our Microsoft-hosted accounts because our Comcast IP is listed on the *residential*  Spamhaus PBL.

5.7.1 Service unavailable, Client host [73.251.103.77] blocked using Spamhaus. To request removal from this list see https://www.spamhaus.org/query/ip/73.251.103.77 AS(1450) (in reply to RCPT TO command)

The instructions at Spamhaus are to email support_biz@cable.comcast.com - an account that doesn't exist.  For residential customers, the key is to complete this form, which results in an email letting us know our IP isn't listed:  https://spa.xfinity.com/report

Business internet support and these forums historically say to contact the Security Response Team at 888-565-4329.  The SRT says this goes to the business internet support team.  I've called them enough that I've unfortunately memorized my 16-digit Comcast account number.

Does anyone have a clue who to connect with?