Changer text before output

You should use a search and replace tool like this instead:
https://interconnectit.com/search-and-replace-for-wordpress-databases/

Althernatively, it kinda depends what you are changing, but you can add regex to your functions file in a function like this:
$html = str_replace( ‘514-555-1212’, ‘514-111-2222’, $html)

I don’t want to change it in the database. I want to change it when the HTML is being output. I see your code, but I want to know what Hook or function I call to get the HTML before it is output.

You have to create your own function, or – it depends where the data is already being stored. Maybe in .po file, or as a key-value pair in the options table, or in the posts table.

If it is just in your loop, then write your own function like this:

function numberreplace($html) {
	$html = str_replace('514-555-1212', '514-111-2222', $html);
	return $html;
}
add_filter('the_content', 'numberreplace');

You can see more arguments here:
https://www.php.net/manual/en/function.str-replace.php
or
https://www.php.net/manual/en/function.substr-replace.php