The answer depends on the situation.  The Telephone Consumer Protection Act says that companies, including debt collectors and your creditors, can't call your cell with an auto-dialer and if they do there is a penalty of $500 - $1500..... And that's for each call! 
There are 2 exceptions:
1.  The Calls must be from an auto dialer
2.  You can't have given them express consent.  (The creditor or debt collector get your express consent in the following way:  when you applied for the credit card or bought the thing they are now going after you to pay for and you put your cell number on the application you gave them express consent.)
With the TCPA, the call does not have to be harassing.  Any robo-call they make is against the law.
Contact us if you think a debt collector is harassing you, your family, or calling your neighbors or at work about a debt you owe.