YOUR PHONE IS FAR WORSE THAN THOSE 30 MINUTES.
This study was about mobile phone exposure and causes of brain tumors. So it was a study of brain cancer. That means obviously that the cancer has to be inside the head. For that, the study had examined which side of the head the cancer patient held the phone primarily and whether that side was also the side where brain tumors had appeared, etc. With this focus, it made sense, that they measured the 'time' of how long a phone was at the ear, near the brain.
And if we speak on the phone, the phone is as near to the ear as is physically possible. If we use the same phone to surf the web or send an SMS message or select a song from our play list, then we are 'looking' at the phone, holding it in our hand in front of us, not touching our ear/head, and the phone is far away from the head. Generally, the amount of radiation received decreases in the square of the increase in distance. If your phone is twice as far, the amount of radiation is not one half, it is one fourth. Etc. So it is in our interest to keep the phone as much as possible away from areas where cancer has risk of occurring.
So far so good. Now the bad part. We have far more types of cancer than brain tumors. If it really is true, that prolonged exposure to mobile phone radiation increases occurrence of cancer in the brain, then very likely that will also be true of most other forms of cancer. Now we get to the problem. For most people they carry the phone upon their body all waking hours. It is in a pocket. In our jeans or the shirt pocket or perhaps hanging on a string around our neck, etc. The phone is very likely always in the same pocket regardless of what clothes we wear. Even if it's a ladies' handbag, that is often with the woman or right next to her.
And for those who are not very familiar with the operation of the cellular mobile phone - it may feel like you only use it when you speak on the phone. The natural assumption is that it's 'dangerous' only when you talk on it. No, the cellular telecoms network is designed so, that your phone is continuously sending a signal [literally your phone transmits a signal thousands of times per second] - it is emitting a radio contact [i.e. it transmits radiation] to the antenna of the nearest cellular tower. So even if you never make calls on your phone, it will generate a steady flow of radiation 24 hours a day. I understand [but am not a radio engineer, so this may be faulty and I will correct if I had it wrong] that the amount of radiation from the phone is roughly speaking the same whether you are speaking on the phone, or if there is no active phone call and the phone only maintains a steady connection.
I do know that the phone will make attempts to discover a network if the phone is on the edge of radio coverage. And I understand [but am not a radio engineer, blah-blah-blah] that when seeking a network, the phone radio output [its amount of radiation] is greatest. So for example if you are driving a car in poor radio coverage, the phone will emit a lot of power attempting to contact the network - occasionally lose contact - and then try very hard to re-establish contact, time and again. This drains the battery rapidly, and that is due to heavy radio activity [i.e. more radiation].
Regardless, understand that the mobile phone you now have, whether a brand new Google Nexus One or a six year old well-worn basic Nokia - every second you have it turned on and connected, it will produce radiation. You are exposed 24 hours a day to radiation from your phone, not only the 12 minutes you spend talking on the phone.