Your iPhone might say "extended network" for a few reasons. If you have an iPhone 5s or earlier and you're in an area with LTE coverage, it could be displaying the extended network message. Additionally, if you have an iPhone 6 or later and you're in an area without LTE coverage, it might show the extended network notification. When your iPhone says "extended network," it means that your phone is borrowing the cell tower of another service provider. This can happen if you're in an area where your own service provider doesn't have a tower, so your phone uses a tower from a different provider. It's not something to worry about and doesn't indicate a problem with your phone.