These are the main reasons why DNI is not working on your landing page.
The tracker settings are not correct
Make sure there is a tracker for the source that you are tracking.
DNI will only work if the visitor on your landing page is coming from the source you set in the tracker.
E.g. if the tracker is configured to track “Google Adwords", then DNI will not work if visitor comes to your landing page from Facebook.
Either the phone number on your landing page OR placeholder setting in Call Tracking app has hidden characters
Sometimes when you copy numbers from other applications (e.g. Microsoft Word), it leaves some non-printable "invisible" characters which you can’t see on the screen.
You can double check if your text is having such characters by copying your number text on this link: https://www.soscisurvey.de/tools/view-chars.php
The text in the website and the placeholder needs to match exactly for DNI to work. Having some hidden characters will prevent this match from happening and hence the DNI will not work.
Solution: Manually type out the phone numbers on your landing page (website) AND the call tracking app placeholder setting.