How long must a number be on the National Do Not Call Registry before most telemarketers can no longer call it?

Telemarketers are generally prohibited from calling any phone number that has been registered on the National Do Not Call Registry for at least 31 days. This means that once a number has been on the registry for that time, telemarketers are expected to respect the wishes of the consumers who have chosen to limit unsolicited calls. The correct answer indicates that the established period is beyond the minimum waiting period, encompassing a wider timeframe that ensures compliance by telemarketers and the protection of consumers' preferences regarding marketing communications.

In this context, the other answer choices reflect incorrect durations, as they do not align with the 31-day rule established by the Federal Trade Commission (FTC) for telemarketer compliance after a number has been registered. Hence, the correct response reflects a full understanding of the requirements surrounding telemarketing calls and consumer rights associated with the Do Not Call Registry.