The conclusion would indeed be the idea that the proposals are ill advised. We can tell this is the conclusion since the rest of the argument can ve shows to support this statement. The first sentence tells us that phones are annoying (thus gives us a reasonable cause to not allow them), and the next sentence after the conclusion tells us that using phones on planes would be worse than on other modes of transit,. The final two sentences are an attempt to prove why phones on planes are worse than on other modes of transit.
We know that the statement in the third sentence is a conclusion, since the final two sentences support it. We could then try to see if the third sentence supports the second using a "therefore."
Allowing cell phone use on planes is ill advised, therefore cell use would be more upsetting on planes than on other modes of transit.
Cell use would be more upsetting on planes than on other modes of transit, therefore allowing cell phone use on planes is ill advised.
Since the second version makes far more sense than the first, we can say that the third sentence supports the second. We could try a similar exercise with the first sentence as well.