This will take the headphone/speaker/line level output from a typical laptop or tablet or music player (or even another phone). It will attenuate it down to microphone level, and then C1 will block the DC voltage out of the mic input jack on the destination gadget.
References
| Reference | URL |
|---|---|
| Headphone OUT to microphone IN for mobile phones | https://electronics.stackexchange.com/questions/239708/headphone-out-to-microphone-in-for-mobile-phones |