Getting other people's messages on iMessage after losing my #? Help!

I used to have phone service through T-Mobile but I ran out of money to pay for bills, and T-Mobile canceled my account eventually. Someone else in the area got an account with them and now they have my number. However, all my friends who had me in their iPhones can still iMessage my phone... But when they try to regular text me, the messages go to the person who now has my number.

The same thing happens when their friends try to iMessage her; I get a copy of the message.

How do I fix this?