The code 023 merged Southampton (formerly 01703) with Portsmouth (formerly 01705) into a new single area code 023 with the former 6 digit numbers in each area migrated to 8 digit numbers. This occurred as part of the Big Number Change on 22 April 2000. Numbers within each area can be identified by the first 1 or 2 digits of the number after the code as shown below. Calls between these two areas (except numbers beginning 0 or 1) can be made without using the code.
Numbers |
Area |
| 0XXX XXXX | Southampton |
| 10XX XXXX | Portsmouth |
| 11XX XXXX | Southampton |
| 8XXX XXXX | Southampton |
| 9XXX XXXX | Portsmouth |