Hello Anthony,

While the NumericTextBox doesn't have such a property by default, you can get its input element and add the pattern attribute to specify to the mobile device what characters to expect, in order to give the correct keyboard: 

<telerik:RadNumericTextBox runat="server" ID="NumericTextBox1">
    <ClientEvents OnLoad="onLoad" />
</telerik:RadNumericTextBox>

function onLoad(sender, args) {
    let input = sender.get_element();

    input?.setAttribute("pattern", "[0-9]*");
}   

Regards,
Vasko
Progress Telerik