The 2 new iPhones have rather dramatically enhanced cellular capabilities. The LTE connectivity is 150 Mbps, up from the 100 in previous Apple phones.
The new phones offer 20 LTE bands, which tops any smartphone in the world. The new phones work on more than 200 carriers around the world. The iPhones also add Voice over LTE, allowing clearer calls and voice and data at the same time, a feature not currently offered by some carriers.
Apple has also added Calls over WiFi, but if you leave the Wi-Fi hotspot you will be seamlessly handed off to the normal cell network.
These capabilities are only for the iPhone and iPhone 6 Plus, so it should encourage some aggressive upgrading when they are released.
Not to rest on those connectivity improvements, Apple has also added 802.11ac Wi-Fi. It’s 3x faster than 802.11n.