As Debian Stretch package repositories have disappeared from debian mirrors, the default upgrade path for Wazo 19.12 is broken.
In order to support upgrade from 19.12 to newer buster-based releases, the wazo-dist-upgrade package has been updated.
That upgrade path also requires manually changing the default debian mirrors to alternative archive mirrors.
For example:
# /etc/apt/sources.list
deb http://archive.debian.org/debian/ stretch main
deb-src http://archive.debian.org/debian/ stretch main