iPhone passcode security

i've noticed behavior on jailbroken (absinthe) ios 5.0.1 iphone 4s , wonder if it's design or security hole:

lock phone (ensure have 4 digit passcode set)
test indeed passcode locked pressing home button , slide unlock (don't unlock it)
use phone place call iphone (i used landline)
answer it
press home
have full access iphone

basically, if know iphone number, can call unlock it.

right?!

 

it must jailbreak bug tried on stock version of ios , when try go home screen asks pass code

 

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