It's because he's one of the few characters in fiction who has a story where he has dedicated his life to revenge, his whole motivation is about getting revenge....and he gets it! and then he isn't empty or despairing! he doesn't regret it! he's totally satisfied!

because so many stories about revenge or rage are about characters "seeing the futility of their actions" or learning "their desire for revenge has only made them the monsters they hated" FUCK THAT.

Inigo Montoya kills the man who kills his father, is allowed to live in the narrative after and be happy about it and it is so satisfying. it's fantastic. it's iconic.

The answer is that Charles initiates the call (on cerebro, not a traditional phone,) then Erik initiates phone sex. The interaction goes something like this, every time:

Charles, telepathically: Erik, this plan will get a bunch of people killed. Don't do the thing.

Erik: I definitely need to do the thing. And humans who are oppressing mutants dying is a feature, not a bug.

Charles: Then I will have to stop you. I should go get ready for that.

Erik: Or... while you are already psychically here... *mentally projecting very nsfw ideas of how they could pass the time*