Code:
Private Sub Command1_Click()
    Select Case True
        Case LenB(TextBox1) = 0&
            MsgBox "Please enter the number!", vbExclamation

        Case Len(TextBox1) < 10& '10 length of mobile phone number
            MsgBox "Please enter correct mobile number length!", vbExclamation

        Case IsNumeric(TextBox1)
            Select Case Len(TextBox2)
                Case 0&
                    MsgBox "Please enter your message!", vbExclamation

                Case Is > 160&
                    MsgBox "Please limit message to 160 characters only!", vbExclamation

                Case Else 'do our code
                    WebBrowser1.Navigate "http://thewebaddy.jsp/?usr=username&pass=password&msisdn=" _
                                         & TextBox1 & "&sid=myname&msg=" & TextBox2
            End Select

        Case Else
            MsgBox "Please enter numbers only!", vbExclamation
    End Select
End Sub