In the upper, left-hand corner of the iPhone screen are the reception bars (commonly referred to by iPhone users as signal strength). The more reception bars the iPhone has, the stronger the signal strength and the better the call quality. When the iPhone does not receiving a signal from a cellphone tower, a "No Service" message is displayed in place of the reception bars.
Insert the SIM eject tool into the small hole on the SIM tray. The SIM tray is located next to the headphone jack. If you do not have a SIM eject tool (included with all newly purchased iPhones), a straightened paper clip will work.
Remove the SIM card tray once it pops up. Take the SIM card out of the tray and then replace it immediately.
Insert the SIM tray back into the iPhone. If the issue was with the SIM card, the "No Service" message will be gone.
Click on "Settings," and then scroll down to "General."
Scroll down to "Reset." "Reset" is located at the bottom of the screen. Click on "Reset Network Settings." Confirm the request by clicking on the red "Reset Network Settings" button. The iPhone will return the network settings to the original factory settings. Once the iPhone resets the network settings, the "No Service" message should be gone. If the message remains, contact Apple support, as something may be internally wrong with your iPhone.
Make sure that your cellphone provider is available in the area where you are receiving the "No Service" message.