This code will restrict letters/alphabet from being typed in the WPForm's Phone field.

wpf-restrict-letters-in-phone-field.php

<?php

function restrict_letters_in_phone_field( ) {
?>
	<script type="text/javascript">
		;(function($) {
			$('.wpforms-field-phone input').bind('input', function() {
			  var c = this.selectionStart,
				  r = /^[A-Za-z]+$/,
				  v = $(this).val();
			  if(r.test(v)) {
				$(this).val(v.replace(r, ''));
				c--;
			  }
			  this.setSelectionRange(c, c);
			});
		})(jQuery);
	</script>
<?php
}
add_action( 'wpforms_wp_footer_end', 'restrict_letters_in_phone_field', 30 );