That is SMS hijacking. That’s why @epitom3 says 2FA with an app.

With one big assumption attacker know you 2FA SMS number, it is not hard to have second number.

For that reason I'm very happy I'm with Giffgaff in the UK - they literally don't have any telephone based support. If you need support you need to message them.....from your account online. Oh and any request to transfer the number takes at least a few days to go through, and you get notification that it's happening - same if you are being sent a replacement SIM. I imagine a number takeover attack would be very difficult due to this.

This assuming you know my SMS 2FA number, it not not that expensive to have second phone number and second sim on a phone.