If this is 1st party collections (ie Sears calling for Sears), there is no real laws that govern them, If this is 3rd party (ie ABC Collections calling for Sears) the you can sue the hell out of them.
The F.D.C.P.A. states that a 3rd party collector can only make contact 3 times in a 7 day period. And it also states that they can not make the consumner incur any charges for the call. Since you are charged by the minute on your cell phone this can be considered a violation as well, it has been tested in some states but I do not have that info.
TG has no idea what he is talking about by the way. There are limites, as I stated, as to how ofter a debt collector can call you.
Also, if you send them a letter asking them to Cease and Desist, you will more than lkely get sued, as that will be the only way they will be able to collect. It does depend on the size of the accoutn as well.
I am an MBA from a major University and I work in International Finance and Credit. I am a CCE and CBF certified through the National Association of Credit Management. Please check your State ad Local Laws.