Nationwide, phone bills are expected to jump by at least £300m as a result of the rises in line rental and weekday calls, which came into force on 1 April.

On 1 February, BT wrote to 10.8 million people on the BT Together 1 package telling them: "At BT, we're always looking for ways to give you value for money", and informing them they would, in future, receive free weekend calls on the Unlimited Weekend Plan.

But the small print in a leaflet contained details of above-inflation price rises.

Monthly line rental has increased by 6 per cent to £11.75, while daytime calls are up from 3.25p a minute to 4p a minute, a rise of 23 per cent. A flat rate charge of 4.5p for evening calls is replaced by a 6p initial charge then 1.5p-a-minute, meaning a 60-minute call that used to cost 4.5p now costs 96p.

Customers can obtain free evening calls by signing a 12-month contract with BT, the country's biggest telecoms company.

"There will be some consumers who benefit from this," said Ceri Stanaway, of the consumer group Which?

"However we think they have sneakily hidden some charges that have affected people, some of whom may not want to sign up to a12-month rolling contract."

The rises follow several surveys in the past year that have criticised BT's service and suggested call centre staff fail to resolve problems.

In an unpublished press release, BT said 640,000 customers had saved themselves £20m by signing a12-month contract since February. That suggests the 10 million who have not done so will have to pay an extra £312m in charges. The rise in line rental alone would make BT £50m, according to the price comparison site uSwitch.com.

"Probably we will make more money out of this," said BT spokesman Michael Jarvis. "But these things are always a balance between a giveaway on one hand and revenue on the other."

He added: "Call charges are incredibly cheap. This is one where a do-nothing, apathetic approach is probably going to cost you money."

One customer complained: "There's just a line that says 'we're making other important changes too', referring you to the small print leaflet.

"And even there, it's not exactly spelt out – even knowing what I was looking for this time round."