There are several possible reasons why your phone might not be showing a signal even though you've inserted a SIM card. Some possible reasons include:
# The SIM card is not properly inserted into the phone. Make sure that the SIM card is properly seated in the SIM card tray and that the tray is securely closed.
# The SIM card is not activated. In some cases, you may need to activate your SIM card before it can be used. Contact your mobile service provider to ask about activating your SIM card.
# Your phone is not compatible with the SIM card. Not all SIM cards are compatible with all phones. If you've recently switched to a new phone or a new mobile service provider, make sure that the SIM card you're using is compatible with your phone.
# There is an issue with your phone's software. In some cases, a software glitch can cause your phone to not recognize the SIM card. Try restarting your phone and see if that fixes the issue.