Hi Charlie,
Mediahawk has written quite a good guide on local SEO and call tracking https://www.mediahawk.co.uk/blog/call-tracking-improves-local-seo/
The article says "You can use DNI numbers to control what number is displayed to a website visitor based on a countless range of conditions. This means the numbers for your social networks can display to those referred from social media rather than applying social media dedicated call tracking numbers to your updates".
I would suggest applying DNI to key web pages on a website for improved tracking.
Hope this helps. Let me know if you have anymore questions.
Zoe-Lee (user of call tracking)