You can buy a new or secondhand phone, either on prepay or disconnected.
A prepay can easily be disconnected, and your number transferred to it, anytime.
Dont ever buy someones contract phone unless it is fully disconnected, and you have given the ESN to TNZ and asked "is this phone 'free on the network?' ".
If you buy new, just get a prepaid - if you ask for a dis/un-connected phone, you will confuse some staff. Unconnected phones are not cheaper to buy - the profit for the dealer comes later from Telecom after the sale is made.