6230 with Prepaid Tracfone sim

[nokia3100]

i just unlock my nokia 6230 phone - i input the correct code and it display "phone restriction off" that's why i'm sure it's already unlocked. but when i inserted the tracfone prepaid simcard - it display "simcard rejected". does anyone know what's going on, why my 6230 won't accept the tracfone prepaid sim? appreciate any help. :???:

phone info:
nokia 6230 (unlocked)
usa - cingular

sim info:
tracfone prepaid sim