You cannot squeeze blood out of a turnip. Collection agencies do not understand that. If you let them intimidate you, then you can sometimes end up making a bad mistake like borrowing money from another creditor just to pay the debt that collector is harassing you about. Typically, when you fall behind on a debt the creditor will try to collect from you for several months.
Then, if you are unable to collect the debt they will turn the debt over to a debt collector who bombards you with harassing telephone calls. Typically, if you find yourself in a situation where you are unable to pay creditors and they are harassing you then you should consider debt settlement and/or bankruptcy options. If you do nothing, then the situation can mushroom into a serious debt problem.