You're not sending the message to the phone, you're sending it to the sim, in the clear. Si I pop this some in my flip phone and receive the code.
I've changed my PIN to something quite long, so hopefully an attacker cannot just pop the SIM card out of my iPhone and use it on another device.
Operators in Finland always give you PIN cards with a preset code like 0000 or 1234, and tell you to switch it in their quick start instructions. I'm sure many people leave it as it is, though. Almost no-one seems to know you can actually set it to be more than 4 digits.
This looks like an area that phone operating systems could fix – by making it easy to change your PIN, and encourage using more than 4 digits for it.
Ah, http://www.techrepublic.com/article/pro-tip-protect-your-and... suggests that you have 3 attempts, then there's a separate system (PIN Unlock Key, PUK) which a comment notes gives 10 attempts.
I'm wondering if you can read the data straight off the card via some physical means, attack that to get the PIN?