I think you have the following situation: (Probably defined by the property designer)
maskedTextBoxPhoneNumber.Mask = "000000 00000" 'Or something similar'
maskedTextBoxPhoneNumber.TextMaskFormat = MaskFormat.IncludeLiterals
in this case your test for
if mtxt.Text = "" then
will fail because the literals included in the mask property are returned in the property Text
you should change the property TextMaskFormat
to
maskedTextBoxPhoneNumber.TextMaskFormat = MaskFormat.ExcludePromptAndLiterals