Although it sounds like an easy question, there has been a lot of litigation over what exactly is considered debt collection harassment under the Fair Debt Collection Practices Act. It's often a question that turns on the unique facts and circumstances of each individual case. But based on the text of the FDCPA itself and the related court decisions, it can be said with some certainty that the following tactics are collection harassment:
Debt collectors cannot threaten violence to collect a debt. This one is pretty common-sense. This prohibition also covers threats against your children, friends, and other third parties.
Bill collectors can't use profane or abusive language. Obviously different people have different definitions of "profane or abusive". But at least one court has ruled that name calling and racial or ethnic slurs are profane and abusive.
Collectors can't call you repeatedly. This not only applies to actual telephone conversations, but also to causing the phone to ring. For example, redialing your number after you've hung up the phone.
Debt collectors must tell you who is calling. In virtually every communication, the debt collector must identify himself and notify you that he is a debt collector. But there is some debate about whether collectors can use a consistent alias. Not surprisingly, many collectors would rather not use their real name when on the job. So some courts have allowed the use of aliases.
Any other debt collection conduct where the "natural consequence" is to harass, oppress, or abuse. This is the catch-all provision. Again, it can be tough to define what conduct has the natural consequence to harass, oppress, or abuse. In some cases, it's easy. In other cases, it's more difficult. Courts have said that mere "bad manners" is not harassment. But the use of words like "liar", "deadbeat", or "crook" probably cross the line and would be considered harassment.