@inject IJSRuntime JsInterop

<input />
<span @onfocusin="@CollapseInputSelection" @ref="@ParentElem">
    <TelerikMaskedTextBox Mask="+1 (000) 000-0000"
                          IncludeLiterals="true"
                          @bind-Value="@CellPhoneNumber"
                          Class="form-control" />
</span>

@* move this script to an actual script file instead of using this hack *@
<script suppress-error="BL9992">
    function collapseInputSelection(root) {
        setTimeout(function () {
            var input = root.querySelector("input");
            if (input) {
                input.setSelectionRange(0, 0);
            }
        }, 20);
    }
</script>

@code{
    string CellPhoneNumber { get; set; }

    ElementReference ParentElem { get; set; }
    async Task CollapseInputSelection()
    {
        await JsInterop.InvokeVoidAsync("collapseInputSelection", ParentElem);
    }
}