That seems to be a musical selection.. so how to tell what is part of the unknown music track and what is hiss? Better would have been recording the quietest environment you could find. Or if the hiss only shows up when recording something, then play a recorded music track. And provide both the phone recording and the original music track so that it can be, more or less, subtracted or distinguished from the hiss.
It would help if the track didn't have interfering music or if you tried recording in quiet, moderate, loud environments, impulse sounds, etc with comments on whether the hiss varies under various conditions. At a guess it's the aggressive ALC (auto level control) when you record with the internal (?) mike. So when there is a quiet part in the music, the ALC cranks up the gain and the noise from the phone circuit becomes more apparent. On that theory it would be good to know if using an external mike alleviates the problem or not.
If it is an ALC problem.. there isn't much that you can do about it.. other than consider using an external mike.. not recording in quiet locations, which could be difficult.. or hope that someone disassembles and tames the ALC.