"According to China.org.cn, 19 families of those missing have signed a joint statement confirming that their calls connected to their loved ones but that they rang out." 

And here's the explanation on CNN - "When phones are disabled or turned off -- which would presumably happen after a plane crash -- calls to those cell phones don't ring, but go directly to voice mail. Friends and loved ones of the missing passengers, however, reported ringing when they called. Technology industry analyst Jeff Kagan says a call would first connect to a network before trying to find the end user, and the ringing sound callers hear masks the silence they would otherwise hear while waiting for the connection to be made. "If it doesn't find the phone after a few minutes, after a few rings, then typically, it disconnects, and that's what's happening," he said."

There's more detail in the article links below.

What would explain the phones still ringing?

Welcome to the future!  Unfortunately, this is not the future of hover-boards whizzing down sidewalks, but rather the future where seeing should by no means be believing.  In this future, even hoaxers with intermediate level skills on standard desktops can produce convincing misinformation.  When big budget production facilities step into the game, how do we know what's real and what isn't?

It's all fun and games, until... It isn't.

See the article below for the "Hoax" info.