Understanding Quad-Band Cell Phones

My questions are:

I understand that Sprint PCS is CDMA, how does this work with quad band/GSM?

Would sprint be able to put me on their network if I buy the unbranded world multi band? Or the quad band/GSM?

Is it possible to have a phone that works for both CDMA and GSM (can I use it on the sprint network and can also put
in my T-Mobile pay-as-you-go SIM or international SIM card)?

First a primer on quad-band: Quad-Band means the phone support multiple GSM bands. We use 850MHz/1900MHz GSM in the U.S. Overseas you find mostly 900MHz and 1800MHz GSM networks. A quad-band phone would support all of these, so you could use it anywhere in the world where GSM service is offered. World Multi-Band is more of a marketing term than a technical one and doesn't have an agreed-upon definition, but typically these phones support the same four bands.

Now to CDMA, which is a completely different network that requires a different type of radio. There are indeed a few phones that contain both CDMA and GSM radios. Verizon sells three "Global Phones," a Motorola a840, a Samsung a795, and a Samsung i830, all outfitted with both radios. Sprint does not appear to offer any such phones with both CDMA and GSM radios.

PalmOne does not make a dual CDMA/GSM Treo, so if the auction you're seeing isn't phony, it's probably a hacked-together version of a two Treos to include both radios. There's no way to know if the phone is legitimate or not, of course, without buying it. 

Since Sprint is indeed a CDMA network, if the phone supports CDMA it will theoretically work on the Sprint network. However, it's up to Sprint to decide if they will allow it on the network: You will have to buy the phone, then call them and ask to have it activated. Since you didn't buy the phone from the company, it's up to Sprint to decide whether to do so, and it's under no obligation to allow the phone on the network at all. Typically, however, this is not a problem.

As for using the GSM features of the phone, this is theoretically possible, but maybe not exactly in the way you've described. There's no technical reason why you shouldn't be able to switch the phone from CDMA to GSM at will, but that would have to be a feature of the handset. Who knows how it's supposed to work. You should ask the seller of the phone, though you might not get a straight answer. This is eBay, after all. Perhaps some of this blog's other readers have done such a thing and can offer some advice from their experiences?