BT has finished upgrading its exchanges as part of efforts to stop scammers exploiting telephone lines which are held open.
Fraudsters used to be able to pretend to be from a bank to con people into giving out financial details.
Now when a person hangs up, an incoming call to a BT line should disconnect within two seconds.
Regulator Ofcom says similar changes have been made to most of the UK's other major phone networks.
Thousands of people have lost money to scammers who have exploited the fact that a telephone line would previously stay connected.
The fraud works when criminals ring someone pretending to be from the police or a bank's fraud department.
To convince the victim they are genuine, the fraudster asks him or her to hang up and call their bank immediately.
But the scammers stay on the line waiting for the victim to make the call, and may even play dial tones down the line to convince them they are dialling out even though the line is still connected.
The fraudsters then "answer" the call and try to dupe the victim into handing over their financial details. They may persuade them to reveal their Pin code before sending a courier to the victim's home to collect their bank card.
In other cases, they have convinced their victims to "protect" their money by transferring it into other supposedly safe accounts.
BT started work to upgrade its exchanges in 2014, and completed the work in September.
Ofcom says all of the UK's mobile operators have now made changes to their phone networks to try to prevent this fraud by reducing the time a phone line stays open to just a couple of seconds.
Most major landline providers have also made the change. BT is the biggest. Its work will have knock-on benefits for the customers of other providers who use its network.
The advice to consumers is to remain wary just in case they are using a phone line which has yet to be upgraded.
They should ring their bank using a telephone number they know to be genuine, ideally from a different phone.
They should also watch out for the latest tactic - "number spoofing".
This is where criminals use software to copy the telephone number of a trusted organisation they want to impersonate, making it appear on a victim's caller ID display on their phone.
They draw attention to it as "proof" they are genuine in order to carry out their fraud.
You can hear more on this story on BBC Radio 4's consumer programme, You & Yours, from 12:15 GMT on Wednesday 4 November.