I have almost always heard the purser announce upon landing at SFO that cell phone useage is prohibited until past immigration. I recall seeing signs on the approach to immigration prohibiting cell phone use. Of course, I have also seen lots of people on their phones walking to SFO immigration.
I have been told not to use the cell phone even past immigration and customs. This was when they wanted you to leave your bags unlocked. I had arrived on an international flight and had cleared customs and immigration. When I submitted my bagage for my onward domestic flight, I was asked if my bags were unlocked. My bags were locked. For a moment, I had a mental block. I told the agent that I could call my wife on my cell phone and et the combination from her. I was told "You are not allowed to use your cell phone until you are past the security".