Why is my SIM card unreadable ONLY on my phone?

My SIM card is unreadable on my phone. I tried taking it out and putting it in the second SIM slot and it is still not detected. I inserted it into my brother's phone and it can be read there. Then, I tried inserting my brother's SIM card into my phone and it can be read, like there's nothing wrong with the SIM card slots. How to repair this? Is the problem with my phone or my SIM card?