Why can't I receive text messages on my Smasung Note 4?

[Android Central Question]

I've just gotten Kakaotalk for my phone, and I've tried making an account for it. But whenever it sends me the verification code, I don't get it. It's supposed to send a text message with a code, but whenever I check my messages it's not there. Thing is, I've already tried changing the message centre number, but it won't let me; whenever I tap on it in the text message settings, it doesn't do anything.
Also, I get messages saying I have missed calls, but I can't get any messages from numbers that I haven't already texted. This is really bugging me since I want to switch to Kakaotalk.