Mobile phone TRS output to microcontroller digital input?
If you make sure to put out a single clean frequency at a time, then a zero crossing detector is all you need.  Audio can always be AC coupled, so a comparator or opamp acting as a comparator is all you need.  One input gets filtered 1/2 supply voltage.  The other gets that as DC bias but also gets the AC coupled audio.  This will make a square wave at the audio frequency.  A micro can measure this frequency and interpret its meaning in the context of a higher level protocol.

Note that this won't work for multiple frequencies sent at the same time, like DTMF for example.