The error that some people receive telling them they have "no sim card installed" isn't really anything new. It's been around probably since the iPhone was released in it's earliest format. Trouble is, there is rarely an easy solution to the problem.
I've experienced this myself with my own iPhone, some days it's not as bad as others, but recently it's been happening more often.
Most threads on the internet explain it's an actual problem with the iPhone itself and recommend restoring the iPhone to factory defaults whereas some explain it has to be taken back to an Apple shop for them to fix. Recently though, I did come across an interesting discussion which concluded the issue could likely be coming from the signal strength to the iPhone. If you live in an area which has a relatively poor mobile signal, do you get the "no sim installed" error message - does it happen if you are somewhere with a great signal?.
I've never really noticed a problem with my signal, although interestingly enough just today I DID notice my signal strength was particularly weak, weaker than usual, and guess what? .... I got the "no sim installed" error message again.
What I read from other threads on the net was to perform a manual reset of the iPhone, which involves pressing and holding the power button at the top of the iPhone and when the screen shows the slider to shut off don't do that but continue holding the power button and then press and hold the "home" button as well until the screen goes black. The iPhone has now powered down completely and when you power it back up again it should have performed a manual reset. Check your signal strength and keep an eye on it - if the signal is good but you still get the "no sim installed" message it would look like a trip to your nearest Apple store, if not, continue to monitor the strength and if you notice the error at some point try figuring if the signal in that area is particularly weak.
It's quite possible I've been duped into thinking this could be the problem, but I've tried figuring everything else and followed many recommendations and nothing has worked. So far, this has, and since performing the manual reset I've not noticed an issue with either the signal or the "no sim installed" error message!!.
Something I have wondered about though, is if I were to take the iPhone back to an Apple store, given the iPhone is now out of warrenty, I wonder if they would replace the iPhone free of charge or whether I'd be looking at the cost of a replacement?!. If anyone knows the answer to that one, I'd love to know
Here's hoping for now though