I'm sure OP has moved on at this point, but I do think it's important to make one thing abundantly clear:
No, it's not a security breach. Regardless of what you might like to believe, your phone number is not private information. Technically, it's not even yours; you don't "own" your phone number. It's a means to get in touch with you, and others are required to know it to even communicate with you via your phone. Much of the same is true of your e-mail address.
By contrast: do people need to know your social security number to communicate with you? Your date of birth? Mother's maiden name? No, they do not. That IS personal information.
Put another way: People can (in certain circumstances) call you on your phone and offer you services, and they can knock on your door and try to sell you things. Whether you choose to answer is entirely your choice. And if you do answer: then whether to trust them or not is your responsibility.
In this case, you lost track of your phone, someone grabbed it, and they used the SIM to obtain your phone number. The same thing could've happened if you dropped a business card with your number on it.