I'm trying to install the unstable FreeSWITCH on debian Stretch, and I get the following error: # apt-get update Hit:1 http://security.debian.org stretch/updates InRelease Get:3 http://files.freeswitch.org/repo/deb/debian-unstable stretch InRelease [4,595 B] Ign:2 http://cdn-fastly.deb.debian.org/debian stretch InRelease Hit:4 http://cdn-fastly.deb.debian.org/debian stretch Release Ign:3 http://files.freeswitch.org/repo/deb/debian-unstable stretch InRelease Fetched 4,595 B in 1s (3,266 B/s) Reading package lists... Done W: GPG error: http://files.freeswitch.org/repo/deb/debian-unstable stretch InRelease: The following signatures were invalid: 20B06EE621AB150D40F6079FD76EDC7725E010CF W: The repository 'http://files.freeswitch.org/repo/deb/debian-unstable stretch InRelease' is not signed. N: Data from such a repository can't be authenticated and is therefore potentially dangerous to use. N: See apt-secure(8) manpage for repository creation and user configuration details. I imported all public keys that I could find, but it didn't help. Which key was used for signing this repo?