My iphone 5s won't let me send SMS?

I opened a new bank account and i received my card, at the bank they told me that to get the PIN code to my card they would send me a text message to my phone (within 24h) asking for a security/verification code that they gave me on a paper, they explained that after getting their text i would have to respond by sending the security/verification code they gave me and then they would send me my PIN code. I got their text message but i haven't been able to send them the security/verification code (a red exclamation point and a message that says "not delivered" appears under the unsent message)

It might be broke