So the general claim is a quad band phone works on both Vodafone and Telecom XT right? Well so far as GSM voice/data and GPRS services go the answer is yes. However if you're wanting full 3G functionality the story is a little different. I have found no manufacturers who actually produce a quad band UMTS/HSDPA handset or OEM module, so what does this mean? Well it depends on where you live and where you want 3G services to work.
Looking at http://www.vodafone.co.nz/coverage/ you'll notice they have 2G/2.5G, 3G and 3G Extended coverage zone maps. The 3G and 3G Extended maps equate to UMTS/HSDPA 2100MHz and UMTS/HSDPA 900MHz respectivily. Telecom XT however provides 3G Extended coverage using UMTS/HSDPA on 850MHz.
So if you purchase a quad band phone will it work on both networks? The answer is both yes and no.
The statment "Quad band" refers to the GSM voice/data and GPRS frequencies, 850/900/1900/2100MHz, however to my knowledge all handsets and modules are currently never more than triband UMTS/HSDPA on 850/1900/2100MHz or 900/1900/2100MHz (yes there are some other frequencies used elsewhere like 1700MHz but this post is NZ specific).
I'll use my SE Xperia X1 as an example:
Phone variants are:
X1i GSM 850/900/1900/2100MHz UMTS/HSDPA 900/1900/2100MHz
X1a GSM 850/900/1900/2100MHz UMTS/HSDPA 850/1900/2100MHz
3G services will work in main cities with both variants on both networks, however I was faced with the choice of either Vodafone 900MHz or Telecom XT 850MHz for 3G extended. One reseller I came across claimed the X1a worked on both networks, which is not quite true.
Keep your eyes peeled for Quad band GSM/UMTS/HSDPA handsets, then we'll finally have a real choice between networks.