Although I've never gone to the effort of actually calling somebody up to thank them, I have gone out of my way to thank colleagues whose code I've been maintaining if I have found it easy to work with.

Code that is: consistent in style and approach (even if it's different to mine) easy to to reason about has clear comments detailing why something was done the way it was

is hard to do consistently, and very often stands out from the spaghetti code that's present in other parts of the system.

The reason I do this is firstly to let the person know I appreciate the effort they have gone to, and secondly to signal to the other developers on the project that this is something worth trying to attain, and at least one person values these things.

Thanks for sharing your story! Now that I'm a freelancer, I see lots of different code bases, and I've seen a few that are really nice. It feels great to see and appreciate someone's else good work. I make a point of letting the people in charge know that the previous/other contributors have done a great job.

One of the great things about joining a well-written project is that usually I learn something new. If that guy was so struck by your code that he called you up, I bet he learned a few things, too.

As in the OP's story, sometimes the code I learn most from isn't immediately obvious, and I have to "go with it" for a bit before I see what's happening. But that takes some openness, perhaps something like what Zen folks call "beginner's mind."

There are times when, like you, I have to start tearing things out and rewriting chunks of functionality, but in general I think programmers are too quick to take that step. My PM anecdote taught me the importance of reading other folks' code charitably, really trying to understand why they made the choices they did, and doing my best to respect and follow their approach. Textual interpretation has the "principle of charity," and reading code probably should too.