I use a SIT at the start of my OGM, coupled with an 8-second carrier-generated voice that says the number has been disconnected, before my call screen function activates.*  It matters not whether a ring occurs before the SIT answers.  Many disconnected lines still ring.  However, my lines do not ring, since I use VoIP.

*The call screen function presents three options -- find me, leave message, start fax, or repeat the instructions.  So, it's double-layered.  A robodialer thinks the line is disconnected.  If it doesn't, it cannot press anything to get through.  Problem solved.  Simply port whatever numbers you are using to a VoIP provider that has a full buffet of options from which to choose.  The more, the better.  Something like Google Voice is not going to work!  Then, point those numbers to be handed off to a new cell phone/ home phone number.  The process will not work by using only one number, although T-Mobile has claimed it is going to introduce some telephony robodial-fighting app in fourth quarter of 2015 or later.  This may help, but, unless it's double-layered, count on robodials getting through quite easily after cracking the system.  Simply, you just have to stay one step ahead of the dialers.  When they're ahead of you, that is when you get the calls.  Quite simple.

Because I have three high-profile "repeater" phone numbers that funnel (point/ hand off calls to) my cell phone, the only calls that get through are "butt-dials" which are harmless and won't normally occur again.  Otherwise, I used to get a robodial about every four minutes during waking hours.  Not kidding.  Anyone with repeater numbers in one of the three largest cities in America knows of this problem.

Good luck.  Now you have been advised of the only way to stop the calls.  For now.  In three or four years, maybe this dual-layered system will no longer work, when the robocalling systems are programmed to dial every digit to get through.  Then, the VoIP providers will make it harder by requiring a two-digit combination before connection.