I truly appreciate everyone’s input, but . . .
As I indicated, review of the Ooma call log confirmed that this was a valid call, i.e. some idiot called that number at 4:20 am. The mystery which remains is why FreePBX identified it as “unknown extension.” I have two analog phone lines of which this is one. One is Ooma and one is MagicJack. I’m too cheap to port the numbers over to a SIP trunk provider. The analog phone lines are managed by a Cisco SPA8800 which I have been using since I first set up FreePBX some years ago. I’m not a VOIP or IT professional – I’m just a somewhat-advanced hobbyist. Getting SPA8800 to cooperate with FreePBX has been, to say the least, a challenge. Even before I updated to the latest distro and switched over to pjsip (primarily), either it or FreePBX had trouble with the CID on an incoming call. According to FreePBX’s Asterisk log, it is still sending events that result in error messages. However, I can both place and receive calls over both analog lines, so that is good enough now for me.
My server is not exposed to the internet. It is behind a router with a firewall.
I don’t know what OpenSSL is, but I will investigate. Unless it is part of the latest FreePBX distro, it is not installed on my PBX.
I have no choice regarding port forwarding of 5060, because it is required for one of my phones to reach my employer’s PBX, over which I have no control. All of my pjsip and chan_sip devices use either 5061 or 5160. I am considering using a VLAN to isolate the VOIP devices from the rest of the LAN, possibly with a cheap managed switch for my office phone.
For what it’s worth, though both Fail2Ban and FreePBX’s firewall have indicated attempted attacks from time-to-time, my system has not yet been hacked, and, per the logs of my VOIP providers, no unauthorized calls have been made. Once again, for me, that’s good enough. I would never consider providing professional services – I’m far too ignorant of VOIP and SIP to do that – so a working PBX is all that I really want, and I appear to have that now.