What to do if someone else has the same phone number as me?

I bought a phone plan and a SIM, chose a phone number among those Fizz presented to me. I activated my SIM, configured my phone and I tried sending text messages. That's when I realized that someone else had the same number as me. The texts my friends sent to me wouldn't all go to me: some of them went to someone else, and that someone else could answer my friends, as if it came from my number. What do I do?

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