Occasionally, this happens by mistake, but there are disreputable companies that switch customers deliberately without their knowledge. A recorded announcement should verify which long-distance carrier is servicing the line you are using. Check your billing statement carefully each month. Avoid contests and other forms that are unfamiliar and require your signature. Your signature could be all a company needs to switch your service without your actual knowledge.
Many companies use telemarketing as a means to entice customers to switch providers.
If you are not interested in switching your service, clearly indicate this to the solicitor. Contact the company at fault and explain you will only pay charges your preferred carrier would have imposed. If this carrier will not drop the additional charges, contact the Federal Communications Commission. Call your preferred long-distance company and report the switch was unauthorized. Ask to be reconnected.