If you’re in an area where you think you should have cellphone service but don’t, put your phone in airplane mode then switch back.  This will cause your phone to re-register and find all of the towers in your vicinity.  This is quicker/takes less energy than rebooting your phone.

Also, when in an area without service, put your phone in Airplane Mode to conserve the battery or optionally shut off the phone.  If your phone is one in such areas, it’s constantly looking for a tower and will rapidly drain your battery.