That's because lock is not the ckeckbox. Lock is the text view containing "Lock SIM card". You need to use hierarchyviewer or uiautomatorviewer to see the layout of the UI.
The UI looks something like:
(0) Linear layout
(1) Relative layout
(0) TextView "Lock SIM card"
(1) TextView "Require PIN to use phone"
(2) Linear layout
(0) CheckBox
So you need to do something like:
linear_layout = lock.parent.parent
check_box = linear_layout.children[2].children[0]
It's not straight forward, but I used this in the past and worked. Please note that the ViewClient version that I used was 2.3. Now it has evolved to 4+.