Nasty, careless and annoying debt collectors are sparking thousands of complaints from consumers.

The Consumer Financial Protection Bureau began accepting debt collection complaints in July and has already received more than 11,000 -- the second highest amount after mortgage complaints, according to analysis from the U.S. Public Interest Research Group.

The most common grievance from consumers: That a debt collector came after them for a debt they didn't even owe. About 2,700, or 25%, of complaints were about this.

Next on the list was harassing phone calls, with 13% of consumers saying debt collectors had called them repeatedly or far too frequently. Another 13% said they weren't given enough information to verify that the debt was in fact theirs or that they owed the correct amount.

Related: Debt collection horror stories

Among the other complaints: that collectors tried to go after debts that have already been paid, attempted to collect an incorrect amount, talked about the debt with a third-party like a family member or neighbor, threatened to take legal action against a debtor or contacted a consumer after being asked to leave them alone.

About one in five consumers who have lodged complaints with the CFPB about debt collection have received some sort of relief as a result -- with 3% receiving refunds or compensation and 19% receiving non-monetary relief, such as stopping unwanted calls. But the majority, or 70% of complaints, were left unresolved.

The CFPB is currently reviewing the complaints and collecting comments from consumers, companies and industry experts about ways the agency could rein in unacceptable practices among debt collectors.