Originally posted by: hytek369
What's the reason?
It depends. Phones sold by a provider (in the US) are almost always "locked" and will only work on that network. Unlocked phones are more expensive, since you don't get the hefty rebates that the cell company throws in when you buy a new phone with a 1- or 2-year service contract.
Also, different companies use different network standards (GSM, CDMA, EVDO, etc.) A phone that only 'speaks' GSM cannot be used -- at all -- on a CDMA network, regardless of whether it is locked or not. Some phones are 'dual-mode' (usually CDMA/GSM) or 'tri-mode' (usually GSM/CDMA/EVDO) and can work on more networks. Generally phones which are sold both internationally and in the US are dual- or tri-mode, while US-only models are more likely to work only on a single network (and to be locked to one provider).
If you buy an unlocked phone that can be used on the network type that Verizon uses, it should work. However, you may not have access to some of the specific features of their network (like online services, etc.) if you do not have a specifically supported phone.
Brief research seems to show that Verizon is using CDMA, and EVDO for data transfer. Sprint appears to be using the same standards, so I would think any "sprint" phones should work on Verizon's network (again, only if unlocked, and maybe not for anything but normal phone service).