It is possible. Amazon Cloud Music takes the entire lock screen. You have to press a lock icon to display the keypad. I am completely stock on my Nexus 4 on 4.3.
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If your roommate has O2 also, try swapping Sim cards and see if that has any effect. That will at least eliminate the Sim card as a potential problem.
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I updated to 4.3 using the OTA zip file. I had no problems. However, I am looking for the autocomplete on the phone dial pad that I read should be there and it is not functioning.
Am I missing something? Anyone have any insight?