Modify the below code according to your requirement.This piece of code worked for me.
int length = 0;
string longestTelephone = null;
for (int i = 0; i < allTelephone.Count; i++)
{
if (length < allTelephone[i].Length)
{
length = allTelephone[i].Length;
longestTelephone = allTelephone[i];
}
}
Hope this may help.