Will a sony ericsson C905 work with a prepaid att sim card?

If the Sony Ericsson C905 is unlocked and free from network restriction, It can definitely be used with a prepaid AT&T SIM card. If it has been locked to any other network, Then it must be unlocked from network restriction to use with your AT&T SIM card.
The safest way to unlock a Sony Ericsson C905 is to unlock using unlock code. You can get the unlock code for a Sony Ericsson C905 from TheUnlockArena.Com at affordable cost with easy unlocking guide. Good luck.