The federal Fair Debt Collection Practices Act (FDCPA) does not prohibit collection phone calls to cell phone numbers nor does it regulate in any way the type of phone the collector may call to contact the debtor. You may be thinking of laws on unsolicited sales calls and the do not call phone list program of the FTC, but neither of those apply to calls from debt collectors. You gave them the number and they are free to call you at that number (or at any other number where you may be reached) so long as they make the calls at reasonable times of the day and do not call you at a place they know that such calls are not permitted (e.g. calling you at work when your employer does not permit such calls). Whether your phone is a landline, cell phone, VOIP, or whatever does not make any difference.
You may, however, contact the debt collector in writing and tell them not to contact you any further about the debt if you want the calls to stop. Once they get that request they cannot further contact you to collect the debt other than to notify you of actions that are about to be taken to collect. And, of course, you may always be sued for a judgment on the debt.
You might also consider bankruptcy if you have a lot of debts that you cannot pay.

