Line Level to Mic Pad Working Randomly With Phones
Looks like R2 needs to be higher. The Android specs call for minimum of 1K from mic to ground.  Your 1K resistor is right at the limit.  An example is 
given for a microphone with a 5K DC resistance.

So, use a larger R2, and proportionally larger R1. (R2=5K, R1=100K)