For curiosity I put a Skinny Sim into a HTC Wildfire phone which according to GSM Arena is a 3G 900/2100Mhz phone.  It doesn't work.  The Skinny site says just 850Mhz and the Telecom site says 850/2100Mhz.  Does 2100Mhz only work on Telecom and has anyone tried to get it working at least in the big cities?

Skinny says
If your mobile device is 3G and compatible with WCDMA/UMTS, 850MHz frequency you can use it on the skinny network.

https://support.skinny.co.nz/app/answers/detail/a_id/215

Telecom says:
Our 3G network, use the 850mhz and 2100mhz frequencies.

If your phone isn't compatible with EITHER of the 850mhz or 2100mhz frequencies, it's likely to receive limited or no reception, meaning calling, texting and data will perform poorly or not at all.

http://www.telecom.co.nz/mobile/mobile/ournetwork/phonecompatibility/