Your phone is dead, you get a call, now what?

It sounds like the adapter is trying to provide the mic in on your phone.

Test by seeing if the mic is suppressed by having only the adapter plugged in but no aux. If the mic works appropriately with no aux, how many black lines are on the aux you’re plugging in? If there’s three, then that aux cable is a TRRS 'requesting" mic control. If you can get one with 2 (TRS), that might resolve it.

If the mic doesn’t work with just the adapter, then it’s the adapter’s fault.