02031 is not an area code in its own right. The area code is in fact 020, and the following 2 digits are part of the local number.
So 02031 should actually be written 020 31, although it makes no difference when dialling the number.
For more information on 020 numbers, see the page for the 020 area code
The cost of calling an 020 number
When calling from a landline, calls to 020 numbers can cost up to 10p per minute, dependent on the time of day, and there is usually also a call set-up fee
When calling from mobile phones, calls to these numbers cost between 10p and 40p per minute.
Most landline operators offer inclusive call packages which allow calls free at certain times of the day, or at all times, and most mobile companies include free any-time minutes in the monthly contract payment, or a bundle of minutes for a reduced cost in the form of a top-up credit.