Forced to double up contact up mobile numbers iPhone

Having an issue since 3.0 update in that I store all contact mobile numbers with an international extension eg. +6421xxxxxx for New Zealand vodafone mobiles. When calls came through as 021xxxxxx the phone still recognised number as the same number. MMS messaging also recognised both. Not anymore...
Now I have to double up my phone numbers for contacts as iPhone doesnt recognise the international extension. Never had this issue on any phone, seems a pretty straight forward whats going on? any advice or info?
iPhone 3G iPhone OS 3.0