WebEx supports closed-captioning in the Meeting Centre only, uses both auditory and visual notifications for events in any session, and is accessible through keyboard inputs in all centres.
An overview of accessibility features in WebEx: https://help.webex.com/docs/DOC-8888
WebEx and ADA compliance: https://cisco-support.webex.com/guest/articles/en_US/Usability_FAQs/WBX18864/myr=false