Hi have the same problem. i have now purchased 6 nokia 1520 phones each of which has failed in the same way. Basically if i ever connect a phone to a bluetooth device the internal mic gets redirected to the bluetooth and after that there is nothing (other getting it replaced) you can do return it to the internal mic. No resetting or firmware changes fix it.
i have a second fault which is caused if i ever place a mic/headset in the 3.5mm jack socket, again the issue is the loss of the internal mic.
In both cases the only way to make calls and be heard is to use the hands free speaker mode.
This issue is reported everywhere, my phones were purchased at different times, from different places in the world, all have failed even when i have not directly connected to a bluetooth headset ie just connecting to another bluetooth device causes it.
Really had enough of this one, i am a developer on the technical preview platform causing me real issues in writing phone apps