Phone-only mode on iPhone
You could turn off Wi-Fi (Settings -> Wi-Fi) and cellular data (Settings -> General -> Cellular -> Cellular Data) to get a phone which works just like a phone. Possibly disable location services (Settings -> Privacy -> Locations Services) as well if you really want to extend the battery life. 

I do believe this would still allow local notifications, though, if you've got any apps that use them (such as some alarm or reminder apps).