Most cell phones kick up their power output when establishing contact with a tower. The "ring" is just the outward manifestation of this (you can have it happen in other occasions, as well). A lot of conference phones are notorious for this. Also, the frequencies used vary by technology GSM is more prone to cause issues with the electronics I come across every day. I'm not sure about avionics packages. I'm surprised that a G-V, especially a USAF one, did not have a reasonably hardened instrument package. At FL500, you get some pretty serious radiation compared to sea level.

Question for the pilots regarding weight and trim: I am on flights occasionally where the pilot seems to back off the throttles pretty significantly coming out of IAH, presumably to trim the bird. Is there a fuel savings, a noise abatement or some other reason for the disparity, or are some pilots just a little lighter on throttle changes than others?