Calls to the non-geographic 03 number are charged at the same rate as calling the Portsmouth 023 number. Where the caller has an inclusive allowance of calls to 01 and 02 numbers, the allowance must also cover calls to 03 numbers. Ofcom rules in place since 2007 ensure this applies on every landline and mobile provider.

This page provides a clear description of call charges for 03 numbers:
https://www.nidirect.gov.uk/articles/contacting-08-and-03-numbers