Cell phones are programmed to look for your cell provider's towers first as a means of connecting your phone to make calls. There are times when the area you are in does not pick up on your carrier's cell towers and your phone will say "No service." Most phones will then automatically look for the next available tower frequency, but not always. Your phone has the ability to manually search for these other cell phone frequencies to connect to.
Press the "Menu" button on your phone from the home screen. For phones that have a manual keypad, the menu button will be brought up by the center navigation button. For touch screen phones, there will be either an icon on the screen or it will still have a button at the bottom of the screen that can be pressed to bring it up.
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Scroll through the icons and select the "Settings" icon.
Select "Network." Some phones will have one step before this. On Nokia phones, you will need to select "Phone" first and it will then say "Operator selection" instead of network. On Samsung phones, the "Advanced" option will need to be selected and then "Network." On most PDA-style phones, network will be the direct option.
Scroll down and select "Manual." The phone will now scan for any networks available in the area.
Look through the list and select the one you wish to connect to. Highlight the name and select it. The phone will then switch its services over to that cell tower frequency.