Three UK networks, EE, Vodafone and Three are still charging customers for devices they already own.
This means that when their customers reach the end of their contract - and thereby own their device - their monthly fee is not automatically adjusted to reflect that the customer has already paid for the device.
At the end of the contract, many customers are unaware that from this point they only need to pay for the calls, texts, and data that they want to continue to use.
For customers with a high-end device, this could mean a substantial drop in their monthly fee.
According to The Citizens Advice Bureau, this has left around 4 million consumers out of pocket and netted around £500m in unnecessary fees for the networks.
Failure to adjust the customer's bill is not a practice of the O2 network.
Citizens Advice have noted that other leading UK networks, such as O2, have already stopped these charges and are urging these remaining networks to follow suit ASAP.