BT customers will have to pay more for early morning calls from 16 February.
BT says about three million customers will see bills rise
About three million BT customers could see their bills rise as it shifts the start of its more costly daytime call rate from 8am to 6am.
Customers on BT's Together Option 1 package will see the cost of landline calls made between 6am to 8am rise from 5.5p per hour to 3p a minute.
BT Together Option 2 customers previously enjoyed free calls at this time but will now pay the new rates.
Those on Option 3 are less affected as they pay a higher monthly fee in exchange for an unlimited number of free calls, each up to one hour in duration, at anytime of the day or night.
BT said about three million of its customers would see their bills rise.
"Our research indicates that most people, if they make calls between 6am and 8am, are on the phone a short time," a BT spokesman said.
"We don't think there will be a big impact on individual customers, but across three million bills even a small increase makes a significant difference for us."
At present, according to utility price comparison website uSwitch, Telewest is the only other provider to apply a daytime rate to calls made between 6am and 8am.