Oops, I missed that your FreePBX is recent enough to have defaults of pjsip on port 5060 and chan_sip on port 5160. You can check this in Asterisk SIP Settings. For pjsip, it’s called Port to Listen On; for chan_sip it’s called Bind Port. If you change these, you need to restart (not just reload) Asterisk.
Assuming chan_sip on 5160, in the SPA change PSTN Line -> Proxy to
192.168.168.139:5160
reboot and recheck. If it still won’t register, use the Asterisk console to set
sip set debug on
and post the REGISTER requests sent by the SPA and any responses sent by Asterisk.
The normal sequence is:
SPA sends REGISTER (without Authorization header).
Asterisk responds 401 UnAuthorized.
SPA sends REGISTER with Authorization header.
Asterisk responds 200 OK.
If Asterisk sees the SPA as registered,
sip show peers
will show its IP address in the host column.