xaml

<TextBox    FontSize="12"
            FontWeight="Medium"
            FontFamily="Montserrat"                            
            CaretBrush="LightGray"
            BorderBrush="LightGray"
            BorderThickness="0,0,0,2"
            Height="28"
            VerticalContentAlignment="Center"
            materialDesign:HintAssist.Hint="휴대전화번호"
            PreviewTextInput="OnPreviewTextInput"
            Margin="0 0 0 20"
            Padding="25,0,0,0">
</TextBox>

xaml.cs(비하인드 코드)

private void OnPreviewTextInput(object sender, TextCompositionEventArgs e)
{
    // 숫자만 입력되도록 설정
    Regex regex = new Regex("[^0-9]+"); // 문자 제외
    e.Handled = regex.IsMatch(e.Text);
}