Look at your SCP-6400 again. Note that it does not have a SIM card. This is because SIM cards are only used in phones that work on GSM technology (in other words, Cingular or T-Mobile networks). Since you say you're sticking with Sprint, look for phones that are advertised with them or Verizon. That way you avoid all that jargon about locked, unlocked, SIM cards, et al.
Sprint/Verizon phones use CDMA technology, referring to a somewhat different modulation process. I have not used a CDMA phone in years, so I cannot speak to how those networks differentiate which phones work to which service provider.
Have Sprint service with SCP 6400 for several years. Satisfied but want to replace phone with more compact, clamshell type. Speaking with various persons and surfing net, find I'm faced with cell "literacy" problems like, "locked","unlocked", "SIMS cards".
Don't want to contract and realize will cost more for phone. Does UNLOCKED mean such phone could be used with purchase of appropriate SIMs card? Does my SCP 6400 have SIMs card that is usable? Are phones available on market that are LOCKED with Sprint card installed? Any advice on what I am really searching for would be greatly appreciated.