Debt collectors can be aggressive when trying to contact you and talk about the money you owe them. They may call you multiple times a day and at different numbers.
The Consumer Financial Protection Bureau explains federal law does protect you against harassment from debt collectors, but it is hard to know when they cross the line from reasonable to unreasonable.
Federal debt collection laws do not give specifics on how many calls equal harassment. The law only says that calls cannot be repetitive with the intent to harass. It can be quite difficult to prove intent. Multiple calls may be ok under the law if you never answer or the debt collector has no way to leave a message.
On the other hand, if you speak to the company or they can leave a message, then calling you multiple times in one day may be harassment. It will likely depend on the circumstances. In addition, if other people find the company harassing, then that could help your case as well.
Since it may not be easy to show a debt collector is harassing you, you can exercise your rights to stop the calls. Federal law gives you the right to tell a debt collector not to contact you via telephone. You should make the request in writing.
The debt collector can then not call you except to let you know they got the letter and will cease calls or to let you know they are taking a specific action, such as filing a lawsuit. You should not that stopping calls do not erase the debt. The company will probably resort to mailings instead.