If you are able to make an assumption that all emergency numbers will be a fixed number of digits, in most cases; 3 digits, then the problem is easier to solve. If so, then they can be ignored.
Otherwise you could process a second list of unique prefix numbers:
After your code:
while ((input = Console.ReadLine()) != null)
{
phoneList.Add(input);
}
List<string> prefixes = new List<string>();
foreach(string number in phoneList)
{
prefixes.Add(phoneList.SubString(0,2));
}</string></string>
Then check that list contains the phone number being validated.