All debt collection agencies are governed by the rules laid out by the Fair Debt Collection Practices Act (FDCPA). This tells them how, when and for how long they can continue to try and collect a debt from you. Knowing these rules can be extremely helpful when dealing with an unscrupulous debt collector and used in your favor if they are breaking said rules.
The following information, while a bit dry, can help you to know when a collection agency is being fair and honest and when they are not. Citations are provided so that if you have to mention the laws when dealing with a collection agency you can do it specifically.
1) An agency cannot force you to pay more than you owe or misrepresent the figures that you owe. [15 USC 1692e] § 807(2)(a)
2) They can’t add fees, interest or other expenses that were not included in your original debt, loan or credit agreement.  [15 USC 1692f] § 808(1)
3) They are not permitted to call continuously, repeatedly or at times that would normally be considered inconvenient. They also can not call before 8:00 am and after 9:00 pm. [15 USC 1692d] § 806(5)  & [15 USC 1692c] § 805(a)(1)
4) Using obscene or abusive language is prohibited and can be seen as harassment. [15 USC 1692d] § 806(2)
5) They may not threaten you with bodily harm for any reason. [15 USC 1692d] § 806(1)
6) Collection agencies may not threaten to take any actions that they can’t or won’t take, such as garnishing wages, taking you to court, taking your personal property or putting negative marks on your credit report. This includes actions that they can take but don’t intend to take. [15 USC 1692e] § 807(5)
7) Without your written consent they cannot inform any 3rd party about your alleged debt, no matter who they may be. [15 USC 1692b] § 804(1)
8) They must send you a written debt validation notice within 5 days of the first time they contact you about a debt. This letter has to include the name of the creditor, the amount of debt owed and a notice of your rights.  [15 USC 1692g] § 809(a)
9) If you let them know that your employer doesn’t approve of their calls they may not call you at work for any reason.  [15 USC 1692c] § 805(a)(3)
10) They can’t ignore your written request (by certified letter) to verify the debt and stop trying to collect after 30 days.  [15 USC 1692g] § 809(b)
11) After they get your certified letter of dispute they must stop trying to collect until they have verified your debt. [15 USC 1692g] § 809(b)