You've a couple of options in that case. Below are a few I can think of off the top of my head.

You could do as suggested in my last reply and use an integer field to store it or a text field with validation only accepting integers then format its display using a code field so you can use PHP to format it however you want.

You could also just improve your SQL to do a REPLACE and replace any invalid characters on `cb_mobile` then use text field with input mapping.

You could use a text field with input mapping then on profile save push it to a separate integer field with the formatting removed, but this will result in double storage.

Yes, but you should check the performance of that query first. You probably will want to add a index for that column to CBs table as well. Depending on MySQL version you can probably use REGEX_REPLACE to simplify that further.

 Now I am going to try to register an unexisting user to joomla through node.js. I guess I can do it directly through DB ?

I doubt it as you wouldn't be able to generate the password matching Joomla's hashing. The easiest way to create users externally is to either use Joomla 4 built in API with authentication token (this will only create the Joomla user and not CB user though) or create an auto action to create users then HTTP call the auto action from node.