If there is a whitespace in the Mask of the control and you try to paste a value containing whitespace, the control will trim the user input. Example: Mask="(###) ###-#### User Input: 1234567890 The MaskedText should be: (123) 456-7890 If you copy this input and try to paste it into a control with the same Mask you will get (123) 45-6789 which is not correct. Possible workarounds !!! 1. In a scenario when the user will enter only digits in the MaskedTextInput for a phone number is to restrict the input by setting the following mask. <telerik:RadMaskedTextInput EmptyContent="Enter digits" Mask="(d3) d3-d4" /> 2. The second workaround is to use non breaking space in Mask, this way there won't be a match between space by the user and space in the mask symbols.