Cannot activate iMessage

Not too long ago, my girlfriend gave me her old iPhone 4S since she had gotten an iPhone 6. She changed carriers and numbers so she left the old SIM card in the 4S but her contract had expired so there's no longer any service.

Today, I wanted to activate iMessage on the 4S, which I had never tried since receiving the phone, while it was conencted to WiFi. She had never logged into iCloud on it, but she did still have the normal Apple ID linked with it, so I went ahead and removed it, and signed in with my own Apple ID which I had created today as well. I also proceeded to sign into iCloud with my Apple ID.

I went into Messages in Settings and signed in with my Apple ID and there I got stuck on "Waiting for activation" which took a while so I started browsing forums to see what could help me out, but nothing yet ...

Basically the real problem is that when I go see in the settings which accounts are allowed to receive and send messages, I see my Apple ID with a checkmark and I am able to remove it, whereas the old phone number is greyed out and has a spinning circle beside it.

I don't know what to do anymore. I restarted the phone, resetted network settings, removed the SIM card, logged in and out of my Apple ID, played with the FaceTime settings ... and still nothing!

Could someone please help me out? Thanks!