Loop won't concatenate a string more than once - vb.net

Question

I'm currently doing a soundex system for homework. For the assignment, I need to replace certain letters with certain numbers. For the assignment, I'm using a do loop. The problem is that only one letter in the string is replaced with a number, while the rest of the letters besides the first one are deleted. For example, Robert is supposed to come out as "R163," but it instead comes out as "R300". I was wondering what I'm doing wrong. My code is:
Private Sub btnCompute_Click(sender As Object, e As EventArgs) Handles btnCompute.Click
Dim word As String = txtInput.Text
Dim first As String = txtInput.Text.Substring(0, 1)
Dim rest As String = txtInput.Text.Substring(1, word.Length - 1)
Dim test As String = ""
Dim combine As String = ""
Dim i As Integer = 0
Do
Select Case rest.Substring(i)
Case "a", "e", "i", "o", "u", "h", "y", "w"
test &= ""
Case "b", "f", "p", "v"
test &= "1"
Case "c", "g", "j", "k", "q", "s", "x", "z"
test &= "2"
Case "d", "t"
test &= "3"
Case "l"
test &= "4"
Case "m", "n"
test &= "5"
Case "r"
test &= "6"
End Select
i += 1
Loop Until i > rest.Length
combine = first & test
If combine.Length < 4 Then
Do While combine.Length < 4
combine &= "0"
Loop
ElseIf combine.Length > 4 Then
combine = combine.Substring(4)
End If
txtSound.Text = combine
End Sub

Answer 1

Trying to keep things as similar to your example... This should work.
Dim word As String = txtInput.Text
Dim test As String = String.Empty
Dim combine As String = String.Empty
For i As Integer = 1 To word.Length - 1
Select Case word.Substring(i, 1)
Case "a", "e", "i", "o", "u", "h", "y", "w"
test &= ""
Case "b", "f", "p", "v"
test &= "1"
Case "c", "g", "j", "k", "q", "s", "x", "z"
test &= "2"
Case "d", "t"
test &= "3"
Case "l"
test &= "4"
Case "m", "n"
test &= "5"
Case "r"
test &= "6"
End Select
Next
combine = (word.Substring(0, 1) & test).PadRight(4, "0")
txtSound.Text = combine