Passcode locked iPhone may still make calls
We have known for awhile that many users can answer calls on their bluetooth earpiece even though they have the phone keypad lock set to ON. Moreover, handsets like Samsung GT-S5600 with keypad lock ON that have a synchronised bluetooth earpiece can call the last number received (LNR) or the number of the last received text message by simply pushing the key on the earpiece. Even if the user isn't using bluetooth earpiece, many handsets (eg Nokia 6303) with a locked keypad can still allow the user to answer a ringing phone.
If you have a passcode lock on your iPhone it is possible, where the owner haven't yet changed the voice default security setting from when it was purchased, to still be able to make phone calls or worst still it can allow a thief (if your iPhone is stolen) to make calls with the handset. By switching the default setting to OFF can help prevent this occur.
The image courtesy of The Register
The passcode lock vulnerability has been reaffirmed in a recent article published at the technology website, The Register: http://www.theregister.co.uk/2011/03/04/iphone_voice_dialling/.