applications – iPhone 4 – Verification required

I have been using the App Store through my Apple ID for a while and everything has been normal.

Now, suddenly, when I want to update an application or download a free application after entering my password a pop-up message titled:

Verification Required

asking me to verify my payment information. In my phone account settings, payment information is set to NONE since always, there is still a red message that says:

You must verify your payment information before making purchases.

The point is that I don't have any payment information, that's why I only use free applications. What to do?