Sometimes Google Ads will not verify third-party tracking numbers because their bots cannot see the number when they crawl the site. In order to bypass this, you'll need to link your Google Ads account to the Search Console inside of Google Ads. Having two accounts linked bypasses the need for the bots to crawl the site.
If you do not have both Google Ads and the Search Console linked, Google is going to need to verify the ownership of the number by going to your website and comparing numbers. If your numbers do not match (which they likely will not if you are using CallTrackingMetrics), they will not approve the ad.
We recommend having separate numbers for Google Ads and ad extensions, and we also recommend having a designated number for your call-only ads. This will create flawless and easily manageable reporting.
For more information about Google's phone number verifying process, see their support page here.