I bought an unlocked version of the K790a on April 27.  I like it very much but yesterday, June 20, it suddenly displayed the message "Insert SIM" when I was trying to make a call.  I turn it off, checked and cleaned the SIM card and battery contacts.  Then I tried to turn it on but the phone refused to do so.  A little red light in the infrared port flashes once or several times in every attempt to turn it on.