Disable voice control on iPhone 4s, ios7.1
There is a way to make home button not to work wheb the home is lock.
First activate Siri
setting >General> Siri > ON
Then
If iphone 5s (may be it work in iphone 4a)
setting >Id&amp;passcode> there is an option where it tell to siri off when phone is lock. 
make sure you put it off.
Thats it home button wont work when phone is lock.
Preventing voice control to make stupid random call