First and foremost, allowing the use of phones during the test is a kind of trust for the students. By the day that phones can be allowed during the test, we can say proudly that both the honesty of students and the trust between teachers and students win a higher achievement.
Additionally, nobody can guarantee that no emergent call will be received during the test. So maybe the students need to do something more important than the test. For example, a terrible accident happen to the father of a student in the testing. His father wants to see his son at his last breath. Obviously, missing the call will be the biggest pity in the student's future life .
I can see no benefit to allowing phones during exams. In a exam setting they are basically cheating machines. Phones could be used to search the Internet or text friends to find out the answers.
The argument that they are needed as a way for parents to contact their children about emergencies is invalid as the parent can contact the school.
I suppose I can't see the value in allowing phones during tests: would it not be possible for students to use it as a mechanism for researching every answer? Indeed, there would still be a need for comprehending the material and analyzing it, but factual recall can not only facilitate that analysis, but is often what is graded most stringently. There may be value in this type of open-material test in some instances, but overall I think the emphasis of schooling ought to be on factual recall in conjunction with analysis.