The iPhone system interface uses Helvetica or a Helvetica variation, much to the joy of font aficionados everywhere.
Helvetica is the font used for the system interface of the original, 3G, and 3GS iPhones. Helvetica Neue, a subtly updated font, is used on the iPhone 4, as was first noticed by the consistently great DaringFireball. Furthermore, older iPhone devices running iOS 4 continue to utilise Helvetica as the system typeface, as noted by DaringFireball, and this shift is specific to the iPhone 4 display rather than the iOS 4 operating system.
Prior to the iPhone's release, iPod models used either Chicago, Espy Sans, or Myriad; following the iPhone's debut, Helvetica was used.
Quoting from "iPhone Who Designed iPhone Font Used iPhone Ringtones" on Every iPod
It is now San Francisco for iOS9. For additional information, visit http://developer.apple.com/fonts.