<telerik:RadMaskedTextBox ID="txtCountryDigits" runat="server" SelectionOnFocus="CaretToBeginning" PromptChar="_" Width="50px" Mask="+###" ValidationExpression="+\d{1}.d{1}.d{1}" MaxLength="4" />
<telerik:RadMaskedTextBox ID="txtPhoneNumber" runat="server" SelectionOnFocus="CaretToBeginning" PromptChar="_" Width="116px" Mask = "(###) ###.####" ValidationExpression = "\(\d{3}\) \d{3}\.\d{4}" />
<telerik:RadMaskedTextBox ID="txtExtension" runat="server"  SelectionOnFocus="CaretToBeginning" PromptChar="_" Width="100px" Mask="x##########" MaxLength="11" />

This only affects the Edge browser as far as I can tell.

The Country Digits and Extension only allow one character to be entered when the MaxLength is set equal to the number of characters in the mask.  You can only enter 1 character if you tab into the control.  Click on the control with the cursor does not allow the entry of any digits.   Removing the MaxLength property allows entry of the full input.