You want to set the input’s type to number for numeric validation.
And use the input’s value to set a default value.
<input type="number" id="title" name="my_option_namePlugin option for input box validation" value=123 />
If you just want a hint displayed to the user inside the input, use placeholder.
<input type="number" id="title" name="my_option_namePlugin option for input box validation" placeholder="Phone Number" />