I would not trust any phones at all. I have yet to see one that can call but that does not contain one proprietary part or another (including the SIM itself). If it exists, you could run Android on it. Android can be compiled from sources, so it's relatively hard to build secret backdoors in. Though even then, I wouldn't trust the mobile network itself, so I'm not sure how much use this is. Besides not using a phone at all, my recommendation is using a simple, disposable phone that is powered off most of the time (battery out). Be especially sure to turn it off when you're within 10 kilometers from home or other places you regularly visit. The mobile network tracks your location at all times, so you have to be careful to prevent it from seeing patterns in your location.
Mobile devices also have another use: they are generally always with you so can be said to be relatively tamper-proof. Laptops are often unattended for hours. Another talk at the 30c3 was about tamper protection and recommended using nail polish with glitter and then photographing that for later comparison. A phone would be a good place to store such photos.
I'm not sure how to disable any phone from using any networking capabilities (which is what you'd want if you only use a phone for these things and not for calling), but never inserting any SIM-card and having airplane mode always on seems a good start. You can still exchange data with it by hooking it up to your (Tails) laptop and just transferring files, so it might still be useful for composing e-mails or reading things.
This won't prevent people from physically threatening you and extorting passwords and information that way of course, but at least you have some idea of what they know and that you are being spied on at all.