What do you think about the time I used to write down rude customers phone numbers and subscribe them to random call services when I worked at a pizza place?
What exactly did you not like about these customers?
They were cheap pricks, if they don't know how to pay, they should pay later.
They may have behaved rudely due to a tragic event of some sort. It's wrong to give out their numbers so that they get hounded for long periods of time as they may have just been in a bad mood.
They are cheap assholes, they don't deserve respect.
It doesn't matter if they did something wrong, you don't fix it by doing something illegal. If they find out that you are using their phone numbers this way, then you could get into a lot of legal trouble.