A trackable phone number is a phone number used for tracking calls generated by different marketing channels and sources. Trackable phone numbers are notably used for tracking calls on a website via DNI (Dynamic Number Insertion).
When a trackable number is used for a call by a website visitor, it passes through a call tracking service server and the call is redirected to an usual company phone nunber.
Call tracking may need many trackable phone numbers, in particular for DNI.
Trackable phone numbers have beeen used for a long time in "traditional" or offline marketing.