I just got this brand new ZTE MAX XL (Sprint version) in Dec 2018. Overall, I am quite happy with the phone. I use Plantronics bluetooth with the phone. Yesterday, I called my friend without the bluetooth and she couldn't hear me. I could hear her just fine. I switched the mode from handset to speaker and same result. When I connected via the bluetooth, she was able to hear me.
I tried calling via Hangout, Whatsapp. Same outcome. She couldn't hear me. It didn't matter whether i called her or she called me.
I researched on the issue for last 9 hours and noticed others had similar issue. I disabled Mic permission in the Google app (similar to disabling Ok Google), but it didn't work. Restarted the device several times to no avail.
I recorded video with sound and there the sound was fine. So the mic was working, just not in several other apps (stock phone app, hangouts, whatsapp, etc.)
This is what fixed the issue for me. I toggled the Do Not Disturb setting under sound. (Settings --> Sound --> Do Not Disturb). It was set to Never. I just toggled it to Always and then Scheduled and then back to Never.
Now my microphone works just fine via the phone app and people can hear me ok either via handset mode or speaker phone mode.
My phone is on Android 7.1.1, build B12 and security patch level Aug 1, 2018.
Hope this finding benefits others.