There really sounds like there is something missing from your question. Lenders can call a loan for a few reasons (late payments, unpaid taxes, house sold or transferred) but not because they couldn't sell it to someone else. If the loan isn't funded yet, then sure they aren't really calling it they can just not fund it. If this is some sort of short term financing (90 days or whatever) then they can simply call it when the term is up. However, if this is a long term loan then no they can't call it simply because they can't fund it - but something is missing.
So this, however, brings up a different question - Why can't they sell it? If this is a correctly researched loan with standard terms then they should be able to sell it. If they can't sell it because you lied/ exaggerated to get it somehow then they can probably call it. If they can't sell it due to some mistake they made (didn't check your credit carefully enough or whatever) and you didn't sign anything that wasn't true, then they can't call it - but I find it highly unlikely they just made a mistake. There is some valid reason why they can't sell it and I suspect that reason may allow them to call it also (some exaggeration to get the loan, or something).