AFAICT your two firewall logs – only two? labeled “Filtered Capture” and “SIPStation and Endpoint” ? – both from January 22nd – were not captured at the same time as any of your FreePBX packet captures of failed calls.
For example, in the firewall pcaps, there does not appear to be any SIP 491 Request Pending messages sent by SIPStation to your PBX. But, you can see several 491’s sent by SIPStation in your FreePBX capture of a failed call from January 20th.
However, it is kind of hard to tell, because the firewall log file names are not very descriptive, and it looks like they may not be capturing all the packets eg. “Filtered Capture” implies filtering was done 
Here’s some info on 491’s from @mbradeen over on the Asterisk developer blog – substitute the words “User Agent” for “My FreePBX” and then assume “Asterisk”/“remote UA” is “SIPStation” – otherwise the intro is relevant and fairly accessible (then it gets into the weeds on direct media and the Asterisk testsuite, so unrelated to your issue, but maybe you’ll enjoy it) anyhow:
Most relevant part:
If your UA receives this [ed: 491] in response to sending an re-INVITE then the remote UA is letting you know that it has already send you a re-INVITE that you haven’t responded to yet. If you receive the 491, you can handle the remote re-INVITE if you haven’t already sent your own 491. If that’s the case, then your only choice is to follow the guidance in RFC 5407.
Please reconsider taking some packet captures concurrently on both firewall and PBX during a failed call attempt and double-checking that the firewall is not eating the re-INVITE sent by SIPStation like maybe an overly-zealous SIP ALG might do.
Please also consider disabling T.38 in SIPStation or trying the *323 prefix mentioned earlier today in this topic per the official SIPStation documentation:
Relevant part:
Once signed up, how do I make outbound T.38 fax calls?
All outbound T.38 fax calls need to be sent to us using a prefix of *323 and then either a 10-digit or 11-digit number. For example, if you wanted to send a fax to someone at 5555551234, you would send the call to SIPStation as *3235555551234.
Important
If we do not receive *323 at the beginning of your number, outbound faxing will not work. In FreePBX, you will need to configure an outbound route accordingly. Unless users are faxing from the User Control Panel with a global fax prefix set up, they will need to dial *323 or a special prefix you have set up for faxing.
Have you tried *323 ?