Cloning a modern SIM card is relatively difficult, versus the old analog phones where it was trivial. Odds are, that's not your problem.

Instead, the most likely problem is the increasing density of assigned telephone numbers. My home phone differs from a bill collections agency in exactly one digit (in my phone number, it's a seven, in the collections agency, it's a one). Guess what? I regularly have messages on my machine screaming at me for forgiveness or whatnot about the money they owe. It's all quite entertaining, I suppose, but it does get old after a while. Back in the old days, they took a lot more care to have a larger Hamming distance between assigned phone numbers, but that just doesn't fly any more.

Another interesting data point: my old cel phone was in the 713 area code, same as my home phone. Both got a regular stream of wrong numbers. My new cel phone is in the 832 area code. I don't think I've gotten a wrong number there yet. I assume that the newer 832 area code is more sparsely assigned, and thus has fewer wrong numbers.

The moral of the story, assuming there is one, is that getting phone numbers in the ugly new area codes, while more annoying to remember, are better for avoiding wrong numbers.