yes sprint is working with CDMA phones... but if you arn't sure.. just take off your battery and you will see if there s any sim card inside loacted..
But I can bet... there will be NO ONE.
A unlocked phone is a GSM (phone with sim card) whith doesn't have any restrictings... meaning for exaple.. if you buy a T Mobile, or AT&T phone.. they work with Sim Card.. and they are locked... meaning they will take ONLY a Sim card from their provider.. A unlocked phone doesn't have any restrictions... meaning it will take ANY Sim Card...
A Phone without having a plan is a Prepaid Phone, a Pay As You Go Phone (known in the US especially for Ex Cingular and now AT&T), or Virgin, or Tracefone. Meaning there is absolutely no contract, no credit check, no obligation at all.
Of course the calls will be much more expensive as at a phone with contract.
For example verizon in the US offers prepaid phones for US$ 0.10 a minute and Tracefone US$1.00 (about that I'm not quiete sure), and AT&T it's US$0.50 a minute.
A CDMA phone you can here activate with Claro, Tricom or Centennial.