I had more time today and I believe I determined the issue.
I have an Xlink BTTN which I had in storage. I used it for a number of years till I decided to go full digital.
I basically called into the HT813 and it answered on every ring pattern regardless of the C=xxxx stuff. I have since restored that to the c=2000/4000;
I decided to connect it from the RJ11 coming from the panel to the RJ11 of the BTTN and then the output of the BTTN's RJ11 to the FXO of the HT813. Went downstairs, dialed my apartment and it picked up after the first ring pattern, I heard dialing and 2 seconds later my cellphone rang.
That's when it dawned on me. The Xlink BTTN might be reproducing that ring and the gateway sees proper signals and answers it.
Where as the panel downstairs may not be producing enough voltage. Then again technically speaking makes sense. I am on the opposite end of the building away from that panel. With the wire distance. It might have alot of loss that the HT813 is not seeing sufficient ringing.
I'm now in the market for a REN or Ring Voltage booster or something that can provide a clean signal the HT813 would recognize. I think it would solve my issue.
With the Xlink BTTN being put into the middle of this, I go dial downstairs, it picks up on the first ring. And I have picked random times to test it. Zero issues
With the FXO connected directly to the wall and downstairs when I dial. It will ring and ring and ring and nothing ever picks up. It eventually times out and just cancells.
I have an older 2500 phone with bell Ringers. Seems to ring strong but that's only listening to the audio from my phone as I hit record and go dial downstairs.
I'll do a syslog tomorrow.