There is a law called the Fair Debt Collection Practices Act (FDCPA). This law provides rights to consumers regarding debt collection. The law also advises consumers as to the things which creditors and debt collection companies can and can not do.
They must call during decent hours. Collection agencies are not allowed to contact you at inconvenient hours (such as before 8 am or after 9 pm or even on weekends). Also, they can not call you at your employer if you tell them that is not acceptable. If you have hired an attorney, then all calls must be directed to him or her. They are no longer allowed to contact you directly.
Debt collectors can not falsely represent themselves. When contacted, collectors are not allowed to misrepresent themselves to get you to pay the debt. This can include claiming to be an attorney, government agency or even credit reporting agency.
Contact you once you have asked them not to. You can write to a debt collector advising them that they are no longer allowed to contact you. They are allowed to send a final letter confirming this. Then, they can not contact you in any way, unless it involves legal action.
If you have an instance of any of these situations, you need to report the company immediately for unfair practices, which go against the law.