If you are calling a New Zealand mobile, that is overseas:
Just dial as normal; you don't need to dial a country code. If global roaming is active on the phone, you will be put through in the usual way. 
For example, to call the mobile phone 029 123 4567 when it is overseas, dial 029 123 4567.
The account holder of the mobile phone overseas pays the extra roaming charges.

If you are calling a non-New Zealand mobile:
Dial as you would a normal international call. Dial 00 country code, network number, mobile number. Remember to remove the initial 0 from the mobile number.
For example to dial UK mobile 056 123 4567 dial 0044 56 123 4567

What will it cost to call an overseas mobile?
It depends on the country the mobile is from. You will be charged the applicable international rate for that country.

We have a separate rate for calls to some countries' mobiles. If no separate mobile rate is listed, you pay the same rate as you would for a call to a landline in that country.