There are margin calls on real estate loans - they aren't nearly as capricious but when mortgages are renegotiated banks will usually refuse to offer a new mortgage if the property is underwater - in that case the bank will generally try to foreclose on the property and recoup the lost mortgage balance through debt collection. This happened all over the place in 2006 and led to the housing bubble.

In the United States, very few people are on short-term mortgages, which means that you have the option, but not the requirement to reneg on it.

You should generally only reneg if interest rates are dropping - and interest rates dropping causes an upward pressure on home prices.