In our org we currently use the Javascript version of libphonenumber (https://github.com/googlei18n/libphonenumber) to validate and correct phone numbers in the Salesforce UI, storing the library as a static resource and calling it from an override page layout, a simple Visualforce page used for the lead object. Validation works very reliably but takes 2-3 seconds, and the override page can cause some unwanted side-effects. Validation allows users to call leads using the Google-built Click-to-Dial softphone built into our Salesforce instance, something out Support team is trialling for a wider launch.

If we wanted to validate phone numbers in the background, so that they were properly formatted before a user ever saw them in the UI, would one alternative be to use the Java or Python version of the library to write a phone-validation service on Heroku and use Heroku Connect (https://devcenter.heroku.com/articles/herokuconnect) to sync the data with Salesforce?