Questions on banks calling due on the loan

[Leon Lee]

Hi, 

Could someone help me on these questions about the bank calling due on the loan? I had a conversation with a seasoned investor and got this info. According to him, Suntrust decided not to deal with residential loans anymore during 2008 and decided to call due on the loans from their borrowers. These investors were requested to pay for example $300K in 2 months, etc.. So, my questions are: 

(1) Can banks call due on my loan in the next recession, even when I pay the mortgages on time?

(2) I heard certain loans (Fannie Mae, etc) are protected and everyone can have limited number of such loans (i.e., 4), is it correct? How can I tell which of my loans is owned by Fannie Mae and is protected?

(3) Does that mean when facing two strategies ,i.e., paying off the limited number of loans vs. buying as much as you can for maximal leverage, the second one has higher risk than the first one?

(4) What are the best strategies to protect assets against such adverse situations?

Thank you in advance!

Lee