Did not reboot the modem because I could see that turning it off and on allowed outbound to work or not (just without audio) - not a double NAT situation - it’s the only router.
Set up Port Forwarding of UDP 5060 (SIP) and UDP 4000-4999 (RTP - The Ports we use - and set in SIP Settings in FreePBX) and no difference .
Had the same problem a couple of weeks with a Cisco/Comcast Router/Modem where the SIP ALG could not be turned off by the user (and Comcast Wouldn’t turn it off - see here: It is so NOT Comcastic!
We have just been replacing these with dumb modems (SB6121) and either Sonicwalls if they will do it, or Cisco RV042’s (or Netgear’s running DD-WRT if they are REALLY cheap) - this always fixes the problem,