The question machine has transmitted a question about cellular networks:

I have been importing small numbers of chinese cellphones that take dual and triple sims.
These phones can handle 900/1800/1900 networks. Is it possible to throw a telecom sim into these phones?

Image credit Steve Biddle

This really is an interesting question. Until about a month ago, there was no question at all. Telecom was on a CDMA network, and Vodafone had added WCDMA 3G to their GSM network. Despite the similarity in naming, there really is zero compatibility between Code Division Multiple Access (CDMA) and Wideband Code Divition Multiple Access (WCDMA) celllar networks. There was no way to take a phone from one network and use it on another.

More recently, with Telecom launching their brand advertising agency’s wet-dream (aka the XT network), with its use of WCDMA and the associated SIM card phones, everyone is rushing to plug XT SIMs into Vodafone phones and visa versa. Their limited degrees of success can be associated with the variation in frequencies being used.

Ignoring the complexities of encoded digital tranmission and cellsite handoff (among others), your cellphone is not too dissimilar to a transistor radio that can only listen on a set frequency. It doesn’t have a dial that you can spin to pick any frequency. If a cellular network does not use a frequency that your phone can hear, you can’t use your phone on that network.

To kick things off, lets tabulate how the two 3G networks in New Zealand use their frequencies (I have a standing invitation for RF nerds to correct me if I’m wrong – props to Steve for corrections so far):

Once you see that table, it becomes pretty clear that the phones in the original question are not going to work on the XT network. It also starts to get pretty damn interesting when you look at some of the wonderphones that Vodafone sell, like the iPhone 3G. The 3G does, well… 3G, on 850 and 2100 MHz. Checking the table, you see that the iPhone 3G is more properly called the iPhone “3G in main centres” when used on Vodafone NZ. The slightly redeeming feature is that 99% of “3G” phones will degrade to GSM (aka 1G, or “before there even was a G”), if they can get it. Vodafone has a legacy GSM network that will support calls and slow-ass internet pretty much anywhere. XT has no such GSM “fallback”.

However, the cold, hard truth is that you’ll have a better experience using an 850/2100MHz phone (such as the iPhone 3G) on the XT network than on Vodafone.

For reasons that I am completely unaware of (RF nerds, help me out!), 3G phones generally come in two flavours: 900/2100 MHz and 850/2100 MHz. The rules of thumb in my HTC Magic review are pretty handy when deciding on which version to use, and also help to explain my table with more clarity:

• Phones with 900/2100 will work great on Vodafone 3G, and barely at all on XT.
• Phones with 850/2100 will work great on XT and pretty well on Vodafone 3G (in main centers), and will probably still be able to do calls (but not 3G) on Vodafone anywhere.

So there you go. If you really, really want to be able to SIM-swap to use 3G on both networks (in main centres), you want a phone that does 850/2100 MHz.

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