Very interesting....

 I am a loop tech, and I had a trouble report about something similar. A customer thought their line was crossed because after the 4th ring, the call sounded like a fax machine picked up instead of going to VM. After line tests came out OK, I did some troubleshooting, and found that the DirecTV receiver was picking up on the 4th ring. I tried to adjust the settings on the DirecTV box with no luck.

 

 Strange.