my iphone 4s keeps getting faulty and missing sim card
The fault comes back quite ofen even after reebooting the phone.
iPhone 4S, iOS 8.2
First, reset the phone by holding the sleep and home button for about 10sec, until the Apple logo comes back again. You will not lose any data by resetting, but it can cure some glitches.
See if there are carrier updates for your device: iOS: Updating your carrier settings
Check the SIM card for any damage.
If this does not help, and to make sure that this is not software related, set it back to factory settings, without using any backup data afterwards. Set up the rest of the personal settings manually and test the phone.
If the phone works, restore from the latest backup.
If it still does not work, this is a hardware issue and the phone has to be serviced by Apple or an Authorized Apple Service Provider:
Use iTunes to restore your iOS device to factory settings - Apple Support