Some states have laws prohibiting slamming.
Unfortunately, some telephone companies use deceptive and misleading telemarketing techniques designed to trick people into giving authorization to switch telephone companies. Telemarketers who are out to switch your telephone company often represent themselves as well-known, established local or long distance companies who are offering discounts. For example, a long distance company may claim to be calling on behalf of your local phone company. Some consumers are slammed even though they tell the telemarketer they are not interested in switching companies. This step won't stop all calls, but it will reduce the number of calls you receive.