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.