I just did this, so I can answer by experience.
I have a SIM800L module that's answering calls via a PHP script on a Raspberry Pi 3, and I wanted to send PicoTTS voice on the line using the AUX jack audio output.
I followed this youtube tutorial using "mostly" the correct values components. I got the resistances pretty much right by playing with 4.7k and 1k resistors, and just the capacitor is shooting up with 47uF instead of 1uF.
I only connected the left side of the jack output leaving the other lead unconnected, the level I get on the phone is alright, the voice quality is alright, I just get some vibrating noise as soon as the phonecall is connected, which disappears slowly and after a while of mostly clean audio comes back, as if a big fly was briefly flying by and then going away again.
But still, I can testify it works, an the noise is OK with me anyway.