Want this random number generator to generate 10 answers and add to a list box - vb.net

Question

I am trying to keep some skills by writing a application during my semester break at school and have found some issues I don't know the answer to.
I am trying to get this code to generate 10 results and concatenate them to a ListBox named lstPhoneNumbers. Here is what I've tried:
Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
'Calculate Random Prefix based upon selected city
If lstBoxCity.SelectedItem.ToString = "Bethany" Then
' Initialize the random-number generator.
Randomize()
Dim Bethany As String() = {"298", "342", "443", "644", "712", "755", "759", "777", "779", "847"}
' Generate random value between 1 and then length of your Bethany array
Dim randomBethany As String = Bethany(CInt((Bethany.Count * Rnd()) + 1))
MsgBox(randomBethany.ToString)
End If
* Edited for Steven Doggart**
Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
'Calculate Random Prefix based upon selected city
If lstBoxCity.SelectedItem.ToString = "Bethany" Then
Dim Bethany As String() = {"298", "342", "443", "644", "712", "755", "759", "777", "779", "847"}
For i As Integer = 0 To 9
lstPhoneNumbers.Items.Add(Bethany(RandomPrefix.Next(0, Bethany.Count - 1)))
Next
End If
End Sub

Answer 1

You really should be using the Random class rather than the old-VB6-style Rnd method. Even if you are using Rnd, you should, ideally, only be calling Randomize once, when the program starts, not every time the button is clicked. To do this with the Random class, first you should create a Random object as a private field on your form, like this:
Public Class MyForm
Private rand As New Random()
' ...
Then, in your button click event handler, you need to create a For loop which generates 10 random numbers, adding each one to the ListBox control as it does so:
Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
'...
For i As Integer = 0 to 9
lstPhoneNumbers.Items.Add(Bethany(rand.Next(0, Bethany.Count - 1)))
Next
End Sub