All numbers need to be stored in the correct format so that when you receive a call if the caller is in your address book their name will be displayed. If you do not store them in the correct format your handset will only display their number and will not show them as being one of your stored contacts and you also may not be able to make outbound calls to them and would hear the message "this call is not covered by your tariff plan" due to the handset attempting to dial the wrong number.
Please also ensure that dial assist is disabled to prevent your handset from formatting the number incorrectly.
Numbers need to be stored with a +44 at the beginning rather than the 0.
The number 0203******* should be stored as +44203******* (not +440203*******)