Edit: After half day, my wife’s Mac started having the same problem. I have then applied the solution that I found in the Apple
How to Stop Waiting For Activation
Once you have set up iMessage it should perfectly without a hitch. However for some users who have either bought a new device or updated their version of iOS, the service can encounter an error and present the following message: Waiting for Activation. This can be a real pain, especially since we would be lost without the ability to text these days, which is why you will probably be eager to fix the problem. Fortunately, it isn’t difficult. Here’s how it’s done:
Before you do anything, you need to make sure you are connected to a Wi-Fi connection as some users have reported that the iMessage service does not activate over a cellular connection. This is not always the case but it is better to be safe than sorry so have a Wi-Fi connection handy just in case.
With that taken care of, there are actually two possible solutions to this problem:
First of all, My Info should be set as your own identity. This usually takes care of itself but occasionally you need to manually give it a kick in the right direction so to check this, open your Settings and go to Mail, Contacts, Calendars. On the next page, go to the Contacts settings and select My Info. Here you can select a contact card to be set as you, so naturally you need to select your own. With this done, your iPhone will know which number and email address corresponds to you and, in turn, where messages should be sent from or delivered to.
However this is not always the cause of the problem but there is another solution you can try:
The cause of many connection problems often boils down to network preferences and you can make it go away by resetting them. Before you begin, have your Wi-Fi router password on hand for reference because you will need to enter it again. As before, open Settings but this time go to General and then down to Reset. Select Reset Network Settings and then reboot your device. Wait a few minutes before trying again and then iMessage and FaceTime should be working fine.
However, if you are still having a problem connecting then it is time to get serious by restoring the original factory settings and then recovering your data with a backup that you have hopefully saved. This should only be done if all else fails but some users have had success with this method. The obvious first step is to back up using iTunes or iCloud, otherwise you will have nothing when you restart your device. Only when this is done should you proceed to open your Settings, go to General, then Reset and finally select Erase All Content and Settings. It will then ask you to confirm the reset and once this is done there is no going back. When it has finished, restore everything from your backup and your iMessage service should be working.
iPhone, iPad, iPod