According to a review of some of the work and issues being addressed by developers working on Android P, the next major update to the operating system, support for a call recording tone will be added. A call recording tone is a beep that sounds during a phone call on the call recipient’s end and is meant to alert them that a call is being recorded. According to the work Google is doing, their particular implementation would play the tone every 15 seconds during a call.
The changes do not add native support for call recording to the Android system as it is already present. However, most manufacturers choose not to enable this feature. This is likely due to concerns surrounding privacy and liability for recorded calls. By constructing the system to detect when applications use a specific API necessary to record calls, Google puts the tools in place for carriers to continue to decide whether to enable the feature via their SIM cards.
Along with improving the framework for recording calls and adding code to support the call recording tone, the commits for Android P indicate that Vodafone Germany is mandating this function be turned on. The reason for this can likely be traced to the fact that Germany is a “two-party consent” jurisdiction which means both parties to a phone call have to be aware of and consent to a call being recorded for it to be legal.
At least one developer commented that the ability to play a tone on the receiving end of a call is not possible with call recording apps on Android. With this addition, developers of these apps may be able to figure out a way to turn on the feature even if particular carriers do not require it as a way to help their users stay in legal compliance.
source: XDA Developers