They won't actually be able to use them throughout the flight. When the in cabin transmitter (a pico cell) is turned off - then their phones won't work. It is generally being stated that the relay transmitter will be off when the a/c is below 10,000 and that it can be turned off at any time - without warning - if the flight crew determine it.
No, I don't like it either but that is what I have read. Having been in voice based telecommunications for 15 years, I still follow the industry.