I suspect the usual way, if not the only way, of buying AGR points is online. Can you purchase points via a phone agent however? If so, what is phone number.
I don't know the answer really, but my 98% guess is no. You don't actually buy those points from Amtrak or AGR. Those points are actually bought from Points.com - which is also why you do not earn the 2 points per $ on Amtrak purchases!