Debian 9 “Stretch” is not yet supported by FreeSWITCH™ . Dependencies remain to be developed and testing reports are needed from the community.
Also, Debian 9 APT has dropped support for SHA-1, requiring our repositories to be re-signed. You will receive errors relating to missing or unusable keys until FreeSWITCH™ version 1.8 is officially released.
For now, deploy on Debian 8 “Jessie” which is a proven platform for production systems. Debian 9 is for TESTING ONLY, so you must determine the dependencies and resolve other issues in the meantime.
See all FreeSWITCH commits here: https://freeswitch.org/stash/projects/FS/repos/freeswitch/commits