If someone transferred your number to another phone by cloning your SIM that's one reason and from what I know you can't invalidate the SIMs you have to kill it by getting the legitimate owner a new number.
it's either that or someone screwed up at the carrier and gave your number to someone else.
Today, i had to get a new cell number because another cell phone had my same number. I had my old number for 6 years, and never once had any problems with it.
How is this possible?
My carrier only recognized my phone with the number, but said that it is common for another carrier to also have the same number registered to another person.