Actually, there is a limit on calls per day. It is in the FDCPA. They are not to call more than once in a 24 hour period. Calling multiple times in a day is harassment and could invalidate the debt.
Actually there isn't and if you had actually read the FDCPA you would have seen that. But go ahead and use that link up there to copy and paste the exact section you are referring to, the exact section that says once a day.