I can't speak about keying a code. However, I was having the same issues with my Omnia - the phone would start popping up this Warning and while the phone would remain fully operational, the warning had become very annoying. I called Verizon and they suggested something which I just now figured out - at least, I think I have the answer to make the warning go away. What is triggering it is that there's a number listed in your Emergency Call List which is also a number for one of your most frequently called Contacts.
To stop the annoying warning, go to the Emergency Call list: Phone > Options > Security tab > Emergency Call. You will then be able to see the list of Emergency Call numbers. Place a '1' in front of the contact's number(s). When you click OK, the phone will prompt you to enter 'yes' for a confirmation and soft reset. Now the phone doesn't recognize the Emergency call number as the same number associated with the Contact and life is without annoyance again.
Hope this helps.