I have used both these services on the same exact device under the same apple ID and password.
I had both my mobile number and also the apple ID as the usernames. I could send and receive under both these.
But this was a mess. I wanted to use my number only. So I just removed my apple ID from the list of user names and started using just the phone number, on my iPhone. Next, I did the same on the iPad. (iPad air) and then for some reason I had to sign out from iMessages on my iPad. Not immediately, maybe a few weeks later. Maybe after an update.
Now at this moment I was using just the phone number. So I tried to log in to imessages on iPad but it failed saying failed to activate. I typed my apple ID and then my password but it doesn't log in. Says 'Failed to activate'. I thought this was because I removed my email ID from the list on my iPhone earlier. I tried to add that to it but it also failed.
Now I'm stuck because the only option i had for my iPad to use iMessages is to use the apple ID since it doesn't have a SIM. What should I do?
Read more: https://ipadget.com/talk/can-t-activate-imessage-facetime-on-my-ipad-air-simless.html