If you're confident (and double checked) that is not a network problem (and you verified the problem is still there when you disable all firewalls and open and forward all protocols and ports, then you want to check your FreeSWITCH configuration.

Stop FreeSWITCH (from console "fsctl shutdown"), then restart it (or watching it be restarted by systemd). On a Debian server, you can now check four most precious files (you will easily find the equivalent files on other OSes):

/usr/local/freeswitch/log/freeswitch.log 
/var/log/syslog 
/var/log/daemon.log 
/usr/local/freeswitch/log/freeswitch.xml.fsxml 

• freeswitch.log this is your primary source, it contains all of FreeSWITCH self-awareness. Also, you'll find here how FreeSWITCH startup has gone. Particularly look for ERR and WARNING lines. For example, maybe you misconfigured one of your sofia SIP profiles, and that profile refused to start. Or, it tried to start but found "its own" network port already taken by another (zombie?) program.

You check this file to see how your extensions have been interpreted by FreeSWITCH: maybe a regular expression you put in a condition was not matching (maybe because what was incoming was not in the format you expected, or because the regex is wrong).

A lot of snafus like: "I set and check this variable, but I forgot to "inline" the set action" can be caught reading freeswitch.log

• syslog/daemon.log contain messages generated by the Operating System, and may report the reason FreeSWITCH has not started
• freeswitch.xml.fsxml is a "brain dump" of FreeSWITCH itself. It contains the entire XML tree FreeSWITCH has built in its memory at startup time from the XML files in conf directory, default settings, calculated settings (eg, local IP address), and expanded macros and variables. Can be most revealing.