P.K rings will turn off phones, on one occassion i sapped my entire phones battery, not sure how though. I can damage the internal mechanisms of a number of devices including watches and pacemakers too.
The best magnetic rings i have found are from http://www.supermagnetman.net/
His N50's are extremely powerful, and compared to my ordinary PK Rings, I have dropped this one many times on tiles, concrete, etc from high distances and still they don't crack or break like normal PK Rings. Although for $35 a pop they are worth it in my opinion
That's where I got mine from. They're great.