The changes have been introduced by Ofcom to protect the future supply of phone numbers in the areas affected and avoids the need to change existing phone numbers.
Ofcom have forecast shortages of new numbers in Aberdeen, Bradford, Brighton, Middlesbrough and Milton Keynes.
The supply of new landline numbers will also ensure that consumers and businesses will be able to enjoy the widest choice of telecoms providers.
Dialling the code does not affect the price of a call and if the area code is not used when dialling locally, callers will hear a free message asking them to redial using the code.
A spokesperson for Ofcom said that changes would be rolled out in the next ten years, but areas with the Halifax dialling code would not be affected for the forseeable future.