how to put messagge box in textbox

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VB

Hai i am creating windows form for staff registeration which this form is connected to database ,i want to put messagebox which if" Please put number in this field" in TELEPHONEtextbox.text .The reason is to inform the user that the data which user type is wrong if their type a string in this Telephonetextbox.text Any advice how i am to do it.Here is my code For your info I am using visual studio 2005 & sql 2005 as database .

class admin
 Private Sub btnregister_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnregister.Click
        If Me.NAMETextBox.Text.Trim.Length = 0 Then
            MessageBox.Show(" Please Enter Fullname")
            Me.NAMETextBox.Focus()
         ElseIf ADDRESSTextBox1.Text.Trim.Length = 0 Then
            MessageBox.Show("Please Enter Address ")
            Me.ADDRESSTextBox1.Focus()
        ElseIf DEPARTMENTComboBox1.Text.Trim.Length = 0 Then
            MessageBox.Show("Please Choose Department")
            Me.DEPARTMENTComboBox1.Focus()
        ElseIf USERNAMETextBox1.Text.Trim.Length = 0 Then
            MessageBox.Show("Please Enter Username")
            Me.USERNAMETextBox1.Focus()
        ElseIf PASSWORDTextBox.Text.Trim.Length = 0 Then
            MessageBox.Show("Please Enter Password")
            Me.PASSWORDTextBox.Focus()
        End If

        Me.StaffTableAdapter.Insert(Me.NAMETextBox.Text, Me.DEPARTMENTComboBox1.Text, Me.I_CTextBox.Text, Me.USERNAMETextBox1.Text, Me.PASSWORDTextBox.Text, Me.ADDRESSTextBox1.Text, Today.Date.ToShortDateString, Today.Date.ToShortDateString)
        Me.StaffTableAdapter.Fill(Me.LoginDataSet.staff)
 
end sub
end class