Unable to retrieve call log programmatically.
First of all, Apple does not officially provide a public API for accessing the call log. This means that you can hack everything you want to access the call log (using private APIs), but when you submit the application to Apple, you are sure that it will be rejected.
Regarding the application you mentioned (quickRemainder)
It gets call log using cvs file or text file. It does not accept information through a calllog db device.
Retrieve the application description.
You can import text or cvs files containing phone calls into this application.
TrueCaller does not receive the call log, it simply uses the application to search for contacts and update contacts with the information they received. They do not read canons
Update: Apple has implemented an infrastructure called Callkit. Although you cannot access the entire call log, you can have some control over calls like
Incoming Call Identification
Call blocking etc.
https://developer.apple.com/reference/callkit
iPrabu
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