What are call extensions?

We’re with Google on this one: for anyone displaying a telephone number on the company website and generally offering consultation, reservation, or sales by telephone, or even just an information hotline, activating call extension is a must – Call Tracking just doesn’t work without them. Potential customers often want to be able to call you quickly and easily without having to visit your website; this makes that process super simple. Call extensions are a vital part of a good customer experience.

Google has a good guide on how to do this:
https://support.google.com/google-ads/answer/7159346

What’s important to know is that you can use call extension at account, campaign, or ad group level. Depending on your account setup, a “deeper” level and “deeper” measurement may be useful. In our experience, it’s sensible to apply campaign level at the very least to provide data for taking meaningful decisions. This pays off when it enables you to determine how many calls come in via your brand campaign and how many come in through “non-brand” channels. Experience shows that more calls are generated via the extension compared with the web site in a brand campaign and vice versa. This puts you in a better position to separate out and assess the actual performance of “non-brand” campaigns with regard to calls made through call extension.

And if you decide to measure in even more detail and generate more configuration work as a result, the best thing for you to do is “edit in bulk.”
https://support.google.com/google-ads/answer/7159348

This enables you to work in a spreadsheet and then upload your edits to save you considerable time in maintaining your many numbers and editing lots of extensions.