userform textbox error trapping

Try this,

Private Sub TextBox1_Change()

    If Len(TextBox1) = 3 Then TextBox1 = TextBox1 & "-"
    If Len(TextBox1) = 7 Then TextBox1 = TextBox1 & "-"
    If Len(TextBox1) = 12 Then MsgBox "Enough Already": TextBox2.SetFocus'goes to the next textbox when enough numbers are entered

End Sub

Private Sub TextBox1_KeyPress(ByVal KeyAscii As MSForms.ReturnInteger)
    Select Case KeyAscii
    Case Asc("0") To Asc("9")
    Case Else
        KeyAscii = 0
    End Select
    
End Sub

The keypress code can be found here for additional details
http://www.cpearson.com/excel/TextBox.htm