So, I had this Windows Server 2012 R2 licence from the Dreamspark program and been using it quite a lot over the years.
After several installations on different hardware (after all, it’s a test lab, always changing), it doesn’t activate. Also, the phone activation option is not there anymore.
The workaround is this: type the serial number in its usual place, and open a command prompt:

slui 4

And voila: phone activation window appears!