A few possibilities:
1) As Habrosus suggested, make sure both SIM card & phone are working.
2) "Incorrect SIM" is also an error message displayed for export sets which are locked to work with certain telcos only. Did you just buy this set? If so you need to go back to whoever sold you the phone as its a locked set.
3) Is your Singtel prepaid card 3G or higher? Unlikely but there are old 2G-only handphones that refuse to work with 3G or higher SIM cards despite SIM cards being a universal design.