The iPhone has been around for a few years now so you'd imagine everyone would be pretty au fait with all of its features.

Wrong. Even if you're a hardcore Apple disciple and have had every incarnation of the iPhone the US tech firm have ever produced, you might not know about these. It turns out there are some secret codes you can key into your iPhone which activate some pretty handy hidden features, according to Digital Spy. Firstly, you can find out exactly how much signal you have got. Yes you can check the little signal balls in the top left of your screen - but they're hardly precise. However, there's a code that gives you a super-accurate numerical reading of your signal strength in any given location. So instead of standing on chairs and hanging out of windows to try and get a better signal, you can easily follow the digits to improve the quality of your calls. Just type *3001#12345#*  into your handset's dialer and press call. Digital Spy say it launches a secret set of menus reserved for developers called Field Mode. Then hold your phone's power button until the 'slide to power off' message pops up and then hold the 'home' button and voila! Your signal should now be displayed as a number. If the number is around -50 it means you have excellent signal and should be able to watch Game of Thrones on your phone without a problem. But anything down near -120 and you're probably going to find it easier sending a postcard or letter. You can also easily hide your Caller ID permanently or on specific calls. If you fancy some old school Bart Simpson prank calls or you just don't want the number you're calling to have your digits (maybe an eBay seller or something) then there's a code for that. You can get permanent anonymity with all your out-going calls by keying in *#31# which blocks your number on every call you make. You might not want to withhold your Caller ID on every call...as none of your friends will probably ever answer your calls. So if you want to be more selective then just enter #31# before keying in the phone number you're ringing and it will hide your number for that call only. Another code hack lets you easily swerve all those unwanted calls. Sometimes you just don't want to answer the phone. You might be watching football or desperately trying to catch up with a boxset and you don't want your mum ringing up harping on about your aunt's birthday or whatever. You could turn your phone completely off... but how the hell would you check Facebook and Snapchat every five seconds? Or you could just key in this handy code which deals with all your incoming calls without you having to hit the dreaded 'decline call' so they know you're avoiding them. Just dial in *#21# to activate the call forwarding function which will send every call you get straight to answerphone so you can get all your last minute exam prep done. If you want to start receiving calls normally again, just key in the same number. But if you never want to miss a call, then you need to key in the 'call waiting' code.  If your phone is always ringing off the hook because you're popular as fuck, you can use this to answer incoming calls even if you're already on the line. You can activate it by dialling *43#. The call waiting function notifies you that a second call is coming in while you're talking on the phone and it even lets you answer the new call straight away. If it all gets too much just switch it off by dialling #43#. Finally, if you're ever trawling your phone desperately looking for your IMEI number in every Settings folder on your phone, stop! There's a code for that too. You need your IMEI number whenever you're changing network, reporting it stolen of taking it in to be fixed. But it can be a bastard to find on your phone.  So instead of spending ages searching for it, just key in *#06# and it will pop right up. You're welcome.