At this moment, it seems there are bugs in the ROM of CM12.1 Android 5.1.
What's the problem?
The built-in MediaServer works unstable related to call recording.
How do we know it?
We checked log files from users, so we know it.
How to know it's true?
No matter what call recorder you are using, the result is the same - when you first power off / power on it works well, but after records several calls it fails to record new calls, so you need to power off / power on again.
How to fix the problem?
No solution from third-party developers. Fortunately the CM team releases new versions of CM12.1 regularly, hope it can be fixed in new versions.