Most people use a passcode to protect their iPhone (if you don't, you should), but few enable the SIM lock. If you're unlucky enough to lose your phone or it's stolen, it's not only the hardware that could fall into the wrong hands. Your SIM might contain data and contacts, as well as allow someone to spend money via text messages. Here's how to enable the SIM lock.
How to stop the iOS 9 hack
Anyone who finds an iPhone with no passcode will be able to access lots of content stored on the device: contacts, music or documents.
However,a bug in iOS 9 has been found that lets anyone bypass the passcode and - if they know the sequence - start sending messages, look at your photos and browse through all your contacts' details.
To stop this from happening (until Apple releases an update which fixes the problem) simply to go Settings > Touch ID & Passcode. Enter your current passcode and scroll down to the section 'Allow access when locked:' and turn off Siri. This only prevents Siri from being used when your phone is locked. You can use the assistant as normal otherwise.
In case you're wondering, the iOS 9.0.1 update does not include a fix for the bug.
How to enable SIM lock on an iPhone
By enabling a SIM card PIN lock, every time your iPhone is powered down and subsequently switched back on again, the PIN will need to entered before the phone can be used, or text messages sent. The same applies if anyone removes your SIM from your phone and tries to use it in a different phone.
From the home screen, open the Settings app. Go to Mobile Data, then scroll down to SIM PIN.
Tap on SIM PIN and you will see a toggle to turn it on or off, and an option to change the PIN (greyed out).
IMPORTANT: If this is the first time you've turned the lock on, you will be asked to enter the operator's default SIM card PIN. You only have three attempts to enter the PIN before the SIM card is locked, and you won't be able to make phonecalls or send texts.
If you don't know the default PIN, search online to see if the information is freely available. For example, Giffgaff SIMs have a default PIN of 5555. You have to enter this code to enable the SIM PIN before you can change it to your own code. Obviously you will need to do this as criminals will know the default PINs for each operator!
When you enter the correct PIN, you will see the toggle is set to enabled, and you can now use the 'Change PIN' option.
Now every time your iPhone is switched on, you'll be required to enter the SIM PIN as well as your iPhone passcode.