Thank you for contacting us.
You have hit a known issue in MaskedInput. It is already logged in our Feedback Portal
where you can track its progress.
A workaround which you can try is to restrict the input of the RadMaskedTextInput to take only digits. To do that you can change the Mask property of the control. Check the following code snippet.
You can take a look at the Numeric format codes
section in the Mask Tokens
help article for further
Give this approach a try and let us know if it works for you.
Telerik by Progress
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