My iPhone 4 was accidentally thrown a few centimetres away on my desk, then few hours later I saw “SIM failure” on iPhone 4. Not sure if it was caused by this incident, because my iPhone 2G and iPhone 3G were dropped alot and nothing wrong with it. However, I only see this SIM failure after it was softly dropped above my desk.
This problem is solved after I restored the iPhone 4 firmware to iOS 4.0.1. Rarely it shows No Service and loose the signal bar and shows SIM failure. But it was working fine after I restored its firmware once again.

I recently experience this problem with iPhone 4. It displays “emergency call” and “different sim detected” screens after I did jailbreak my iPhone 4 using JailbreakMe. This “Emergency screen” is different with previous Emergency Call and connect to iTunes screen.

So, this problem is appear on iPhone 4 after jailbroken using JailbreakMe and unlocked using ultrasn0w. It happens whenever I tried to change my SIM card, even though my iPhone 4 has been activated, full jailbroken (using JailbreakMe) and fully unlocked (using ultrasn0w).

If you having error “emergency call” and different sim detected on iPhone 4, read this guide:
Fix “Different SIM detected Please connect to iTunes” problem on iPhone 4

Anyway, if you see this emergency call screen due to activation, you will need to Activate iPhone 4 using official micro SIM card or Phonebook card