nzgeek: I'm well and truly late to this party, but the info I've got might be useful for others in the future.

The Telecom Y300 is just a rebranded Huawei Ascend Y300. There are two versions of this phone: one that works on the Telecom 3G frequencies, and one that works on the Vodafone/2degrees 3G frequencies. The Telecom-branded phone will technically work on the Voda/2d networks, but in most areas you'll only get 2G/2.5G (EDGE) data speeds. It's a little better than dial-up speeds, but not by much.

In general, the cheaper phones lack the hardware needed to operate on both the Telecom (850/2100) and Voda/2d (900/2100) 3G networks. It's just one of the sacrifices that are made when building to a price point. The Telecom phones will generally operation on the 900/2100 network, because they fall back to 2G  and almost everything has quad-band 2G these days. But the 900/2100 phones won't work on Telecom, because Telecom don't have a 2G network to fall back to.