Upgraded to Yosemite and iOS 8.3, iMessage problems

I recently upgraded to Yosemite. As the system was coming up, and the Message app was started, it asked if I wanted to enable iMessage on my iPhone. I said yes, and it gave me a six digit number to enter. My iPhone was connected at the time and I got a keypad on the screen. I entered the number, and hit enter but nothing happened. I couldn't kill the app, so I had to reboot my iPhone 6 in order to regain control. Then when I tried sending text messages, it said "iMessage not enabled". When I went: Settings->Messages it showed the message "Waiting for activation" under the the iMessage row. When I then select "Use your Apple ID for iMessage", it tries to log me in, but fails with the message: Could not sign in. Please check your network connection and try again.

I found this Apple support URL: If you get an error when trying to activate iMessage or FaceTime - Apple Support

The suggestions of turning iMessage and FaceTime on/off didn't help.

Any suggestions?