Mobile Number Portability (MNP) was implemented in New Zealand in April 2007.
MNP allows mobile users to change networks/carriers without changing their mobile number. This effectively means that you can not know that an 021 number is on the Vodafone network, 022 is on 2degrees or an 027 number belongs on the Spark network so any applications or services that rely on carrier specific functionality need determine the network in some other manner.
Bulletin Connect manages this for clients by subscribing to the IPMS service managed on behalf of the carriers by a third party. All numbers that are ported are listed here soon after they change networks. However, there can be a delay of up to 3 days (usually less) before we are told of the change and can correctly route messages.