The common bandaid solution is to toggle speaker mode for EVERY CALL. It works, but is a pain and leaves the caller confused. I've found that powering on from a completely OFF state causes the problem. A quick reboot is the only semi-permanent fix I've found Perform a SOFT REBOOT: Bring up the power menu with the power button and hit REBOOT. So if you need to poweroff or remove your battery for some reason, turn it on, perform a soft reboot when it comes up. Yes, you are booting your phone twice. I have a feeling toggling airplane mode may fix this for some ppl.