Some variants of the OnePlus 3T allow users to record phone calls natively. This feature is unavailable on the global firmware, but developers discovered that you can enable call recording with just one simple build.prop command.
XDA Senior Member padlad made this process much easier for the average user by creating an app to automatically enable call recording. The initial version required a button tap to activate the feature, but padlad managed to automate the entire process with the latest release. The app works on the OnePlus 5 too as confirmed by XDA Senior Member shadowstep later in the thread. If you are rooted and running OxygenOS as your daily driver, head over to one of the forum threads for more details.