In December 2016, Egypt blocked access to Signal. In response, Signal's developers added domain fronting to their service. This allows Signal users in a specific country to circumvent censorship by making it look like they are connecting to a different internet-based service. As of October 2017, Signal's domain fronting is enabled by default in Egypt, the United Arab Emirates, Oman and Qatar.
As of January 2018, Signal was blocked in Iran. Signal is not currently blocked in Iran. Signal's domain fronting feature relies on the Google App Engine service. This does not work in Iran because Google has blocked Iranian access to GAE in order to comply with U.S. sanctions.
In early 2018, Google App Engine made an internal change to stop domain fronting for all countries. Due to this issue, Signal made a public change to use Amazon CloudFront for domain fronting. However, AWS also announced that they would be making changes to their service to prevent domain fronting. As a result, Signal said that they would start investigating new methods/approaches. Signal switched from AWS back to Google in April 2019.
(Copy from Wikipedia)