Wow, lots of confusion on Preferreds. I have owned Arr-Pa, Amtg-Pa, CYS-Pa, Mitt-Pb, and AgncP. All, roughly 8% when PPS close to par(25.00).
To Mike, a "Call" is advantageous to the Company not the Preferred share holder. Therefore they would not Call the shares if they were trading less than Par. So the Call is at Par(25.00, 50.00 or whatever the initial issue price). So it works like a bond, in that the Call date is your maturity date on the bond, and if you don't have enough dividend potential to the Call date, when examining the PPS, don't buy.
Example XYZ trading at 28.00(par 25) has two years to call. It gives you 2.00/ year in dividends. Should you buy? No. You get 4.00 over two years but might have to give three dollars back. Not good. So think of it as a bond and determine "Yield to Call" before buying.
AGNC doing a BK is not the only way you can lose Par. The PPS can go from 25.00 to 15.00, and stay there. There is no rule that says it has to trade above 25.00. If the common goes down to 20, and the common dividend is .50, it will be the same dividend as the Preferred(.50) but the purchasers today of the Preferred will have given up 11.00 in lost paper capital.