Handoff with different iPhones
The 'Handoff' functionality depends on your iOS version and device hardware, as well as your Mac hardware. As it states in in the Apple support page for 'Handoff':


  Handoff works with several apps that come with OS X Yosemite and iOS 8: Calendar, Contacts, Mail, Maps, Messages, Notes, Phone (iOS only), Reminders, and Safari. It also works with Keynote v6.5, Numbers v3.5, and Pages v5.5 for Mac, and Keynote, Numbers, and Pages v2.5 for iOS. Third-party apps may also work with Handoff.


iOS hardware that is compatible with Handoff:


iPhone 4S (sharing iPhone calls only)
iPhone 5 or later
iPod Touch (5th generation)
iPad 4th gen and iPad Air/Air 2
iPad mini, mini with Retina display and mini 3


Further requirements for specific features are found on Apple's support page.

Mac hardware that is compatible with Handoff:


MacBook Air (Mid 2012 and later)
MacBook Pro (Mid 2012 and later)
iMac (Late 2012 and later)
Mac mini (Late 2012 and later)
Mac Pro (Late 2013)