Since Hangouts will attempt to communicate over tcp via port 80 it seems unlikely that the firewall is explicitly blocking the traffic (the proxy should still forward the traffic to Google to back to your device), but there must be some bigger problem here.

Google G Suite Connect and Access: Google Hangouts About Google G Suite Connect and Access: Google Hangouts Students will learn what Google Hangouts are and how to exchange messages using Google Hangouts. We also cover video and phone hangouts. Google begins evolving Hangouts into Google Chat Jun 01, 2020

Ways to contact teams at Google. Report a safety or abuse issue affecting a Google product If you know of a safety or abuse problem with any of Google's services, we'd like to hear about it right

Google Hangouts is a singular communications platform made by Google to assist users initiate and engage in various communications channels, such as text, video or voice chats. The platform has proven to work in either one-on-one implementation or within a group. Hangouts … How to Make a Conference Call With Google Voice Mar 17, 2020

9 Ways to Fix 'Unable to Fix Call' in Google Hangouts

Jun 08, 2020 Hangouts Error on Android Phone: Cannot connect to the server (feature discontinued) Google Hangouts phone app: How to start a group SMS where one of the recipients is a Hangouts user? Hot Network Questions Was it possible to write a novel on a BBC Micro 16kb/32kb memory era computer without expansions? Google Plus Hangouts - Google Docs