Mobile phone users risk being "held hostage" by their provider because of the cost of switching to other firms, according to a report today.
Consumers are confused about their phones being "locked" to one company, with almost a third not sure if it is legal to change to a different one.
Most of the 2,000 adults questioned by mobile network giffgaff said they should be given information about how to "unlock" their mobile when they bought it.
Mike Fairman, chief executive of giffgaff, said: "Consumers risk being held hostage by their mobile network to the tune of £400 million simply to switch networks with their existing phone.
"The practice of selling mobile phones locked to one network, or Sim-locking as it is known in the industry, is having a big impact on consumers' ability to get the best deal on the market."
Consumers were charged around £17 to switch networks, said the report.