If your iPhone says "No service" even when there is actual service available, it could be due to a number of reasons. Firstly, check if the signal reception in your area is strong enough. Check the coverage map of your carrier, and if you find that your location falls within the map, move to an open area to get better reception.
If you are not in a remote area and the signal strength is still low, then there might be a problem with your iPhone's antenna system. This could be due to a hardware issue or a software glitch. In this case, try resetting the Network settings of your iPhone by going to Settings > General > Reset > Reset Network Settings.
If none of these solutions work, try removing the SIM card and carefully re-inserting it. If the problem persists, it may be time to contact your carrier or Apple Support for further assistance.