Customer complaints about being transferred to another phone company without authorisation have more than doubled in a year.
The Telecommunication Dispute Resolution Service says the aggressive practice appears to be motivated by marketplace competition.
The service received 66 calls regarding unauthorised transfers in the 12 months to March, compared with only 30 in the previous year.
Chairman David Russell says it appears that transfers are being undertaken in some cases when people have only agreed to think about the offer.
If customers receive a cold call offering a better deal than that of their current supplier, Mr Russell says, they should make clear that they are not switching and require further information.