Originally Posted by
ralfp
The OP was asking about Customs, not Immigration. OTOH I assume your error was to correct the question to read 'On what authority is cell phone use banned at immigration.
Many Americans think that the guy fiddling with your passport is "Customs" - thus the common question of "how long does it take to get through Customs at Heathrow" to which the reply is "20 seconds, 30 if you have a limp".