Weird problem with FiOS POTS line

[#1]

I have the following symptoms on my POTS line that are even baffling the Verizon technician:

1. When I dial in (using my cell) to the house line it rings once and then there is a loud crackling sound followed by silence. (The sound actually sounds like a voice announcement system that's malfunctioning.) Occasionally, voicemail answers without the phone ringing at all.
2. When I attempt to dial out from the landline, I get a fast busy signal or dial tone that remains even when I dial the digits.

I've tried resetting the ONT per the instructions from Verizon Support (unplug the battery and power, wait 5 minutes then reconnect them) and the problem goes away only to return hours later. I've also disconnected all the phones in the house and noticed that the problem still remains.

As well, the technician has replaced the ONT and the problem still recurs! I suspect that it's a problem at the CO. Any ideas?

[#2]

I just remembered to add another symptom: the Internet access slows down considerably!

[#3]

My first thought is it's inside your house.  I know you said you unplugged all your phones, but did you know there is a spot in the Ont where it says customer access?  In there there is a telephone jack and a spot to hook up your inside wires.  Take a hardwired phone and plug it into the telephone jack, this should disconnect all your inside wiring.  Now dial your phone number and see if the trouble still exist.

Let me know how you make out.

[#4]

Sounds like a fax machine or phone problem, this is a very rare issue so it's hard to say it is at the CO.