How is it possible to receive a call from a number that is not inserted in a phone?

My phone is a Samsung A20. It fell in the sea. The phone is dual sim and 2 sim cards were in the phone when it dropped in the sea. The next day, I removed sim 2 from the phone. A week later, I took the phone (with sim1 still in the phone) to the technician who was able to fix it in 6 days.

I got my phone back and removed sim 1 from the phone but inserted the old sim 2 back and a new sim1 with different number from the first sim1. However, I have been receiving calls from the old sim1 number (which I had removed) to sim 2.

How is that even possible when the old sim1 is not inserted in a phone?

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