Why can't I use SMS audio after upgrading ios8?

Hello, to use SMS audio service, you need to turn on the iMessage function. The way to open iMessage is: Settings-Information-Click to open iMessage. At this time, you need your apple id login account. In a few seconds, your iMessage will be activated successfully. Next, you can use iMessage to send voice, video and other information to iMessage friends (the other party must also be an Apple device, such as iPhone or iPad, itouch and other devices, iMessage function is also activated, and finally your computer should be connected to the Internet).