~# apt-get update
Hit:1 http://ftp.uk.debian.org/debian stretch InRelease
Hit:2 http://security.debian.org/debian-security stretch/updates InRelease
Get:3 http://files.freeswitch.org/repo/deb/freeswitch-1.6 jessie InRelease [4,628 B]
Ign:3 http://files.freeswitch.org/repo/deb/freeswitch-1.6 jessie InRelease
Fetched 4,628 B in 0s (11.3 kB/s)
Reading package lists... Done
W: GPG error: http://files.freeswitch.org/repo/deb/freeswitch-1.6 jessie InRelease: The following signatures were invalid: 20B06EE621AB150D40F6079FD76EDC7725E010CF
W: The repository 'http://files.freeswitch.org/repo/deb/freeswitch-1.6 jessie 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.