There are a lot of users who rely on Click to Call to ring their extension that has follow me to external phone numbers, same with people who want to use this for external phone numbers. This will break it. Perhaps there should be an optional header, such as:
The caller can be an extension or not, so, validating if is an extension or not will be useless. The idea of the click to call is to implement it with a third party software or your website. Internally, you can define a valid extension or number to be used as caller.