Programming challenge: how to check if all elements in a list is unique

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C#

.NET

Helllo! im doing a c# programming challange and im woundering if i understood and solved the question right?

The question is:
Given a list of phone numbers, determine if it is consistent in the sense that no number is the prefix of another. Let’s say the phone catalogue listed these numbers:

Emergency 911
Alice 95 225 256
Bob 91 12 54 26
In this case, it’s not possible to call Bob, because the central would direct your call to the emergency line as soon as you had dialled the first three digits of Bob’s phone number. So this list would not be consistent.

The first line of input gives a single integer, 1≤t≤40, the number of test cases. Each test case starts with n, the number of phone numbers, on a separate line, 1≤n≤10000. Then follows n lines with one unique phone number on each line. A phone number is a sequence of at most ten digits.

For each test case, output “YES” if the list is consistent, or “NO” otherwise.

I solved it this way:

List<string> phoneList = new List<string>();
         string input;

         while ((input = Console.ReadLine()) != null)
         {
           phoneList.Add(input);
         }

         for (int i = 0; i < phoneList.Count; i++)
         {
             for (int j = 0; j < phoneList.Count; j++)
             {

                 if (phoneList[i].Substring(0,3).Equals(phoneList[j].Substring(0,3)) && i != j)
                 {
                     Console.WriteLine("NO");
                     return;
                 }
             }
         }

         Console.WriteLine("YES");

Is it solved Correctly? and i tried to use the Distinct() method insteed but didint know how to make it work with the Substring() method. I really appreciate all Anwsers! Thank you!