One of the crucial features missing from Stock Android (and several other Android forks) is the ability to record calls. It isn't a feature one would use frequently but comes in handy now and then. Third-party solutions do exist, but they open up the possibilities of malicious code being distributed along with a call recording functionalities. It's becoming harder to trust third-party apps, given the frequency at which they're found to harbor malware.
As absurd as it may sound, recording your own phone calls falls in a bit of a legal grey area. In regions where one-party consent recording laws are applicable, one can safely and legally record their phone calls, as it does not require the consent of the other participant. Things get trickier in two-party consent regions, where a recording can be deemed illegal and a potential invasion of privacy if both parties do not consent the recording.
It looks like we're finally going to get an inbuilt call recorder with Android P, but there's a catch. Some commits in the Android P source code indicate that a 1500Mhz tone will be played every fifteen seconds every time an application attempts to use a call recording API. The purpose of the tone is to intimate the recipient that the phone call is being recorded.
The implementation of said feature lies solely on carriers, and for now, only Vodafone Germany is the only carrier that will mandate it, owing to strict two-party consent laws in Germany. We can expect other carriers to follow suit, especially in areas that mandate two-party consent.
However, there is one minor setback, once implemented by a carrier, there is no way to switch off the sound. Ideally, it should have been something that could be toggled off once consent has been given by both parties to record the call. But, as always, we can expect the developer community to come up with some solution for it.
News Source: XDA developers