Q: iMessage 'waiting for activation' after iOS 9 upgrade

Try going to Settings > General > Date & Time. Turn off "Set Automatically," and then set the time to some point in the distant future. Try to sign into iMessage again... it will fail. Restart your iPhone, then turn "Set Automatically" back on, and try one more time to sign into iMessage. It should now work. If it still doesn't, you may need to restart again.

If this procedure doesn't fix it, see: If you get an error when trying to activate iMessage or FaceTime - Apple Support.

Off is better. If not, you have to turn it off after adjusting the date and then immediately turn it on again.

I have the same problem with iMessage and FaceTime activations on my iPhone 5, but only with my Apple ID that is used on multiple devices. I haven't upgraded my iPad to iOS 9 or my iMac to El Capitan, and iMessage was working well in 8.4.1 on my iPhone up until I upgraded last night. My wife's Apple ID is tied only to her phone, and both her iMessage and FaceTime are activated on her iPhone 4S now running iOS 9.

Seems like the cause of this issue was never completely addressed in the iOS 9 beta/developer forums, aside from all of the troubleshooting advice that has been referenced since iOS 8.

I have tried your advice, to no avail. I also have tried everything posted by D Cohen in the FaceTime activation thread Re: I can't login FaceTime where his suggestions included going through all of the following links, and in short everything from airplane mode/wifi activation to restoring the iPhone has NOT worked for me.

https://support.apple.com/en-us/HT204408

iOS and OS X: Link your phone number and Apple ID for use with FaceTime and iMessage - Apple Support

If you get an error when trying to activate iMessage or FaceTime - Apple Support

http://www.igeeksblog.com/fix-imessage-waiting-for-activation/

Is there anything else we can do to get activation working?

I am having the same issue... tried all of the "fixes" without any working.  Quite a few threads on this now, so hopefully its resolved soon.

This is on a iPhone 6, but I was looking at my buddy's iphone6 to compare and he had a different option available on his phone after he upgraded.  He did OTA and iMessage worked for him. I ended up doing a reset of all settings  Settings>General>Reset>Reset All Settings.  Now iMessage works.