My father's phone had been unable to make voice calls. I assumed the problem was VoLTE, and indeed, it was not enabled. Unfortunately, the menu about it, while there on my phone (the same model, a Nokia 225 4G), was not there on his.
First, I tried swapping the SIM cards between the phones, to no avail. Then, I using a search engine to try and find any information about this problem, I saw someone who suggested updating the phone software.
So I tried to run the updater, but immediately ran in to the problem of the SIM not having any data. I was expecting that, and swapped it out for one that had. I then tried to update. Same error. Checked the data on the phone it came from. No problem there. Searched. Ah, data is enabled through the the quick settings menu.
Finally, I just had to run the updater, and the VoLTE menu appeared, I enabled VoLTE, and everything worked smoothly.