I see you have a poor choice of options, and as a specialist in this sphere, I can provide you with more info (I hope you still find it helpful). The good way to achieve it is by using regular expressions (regex). It’s not that confusing if you learn more.
Many online resources can help you learn about regex and how to use it for formatting phone numbers. You can also find pre-built regex patterns that match phone numbers in the desired format or use uk temp number. Alternatively, you could use a library or plugin that already has this functionality built-in.