Am having the same issue, but I am thinking mine is due to my phone hardware. (Sprint Store ordered me a replacement...2-3days...SIGH)

My current workaround is to place a call and then switch to speaker so I can hear the phone ringing and or person talking. After call is working in speaker mode I then switch speakerphone off and sometimes the headset and mic work other times I have to switch back to speakerphone and try again.

(I tried the hint of disabling the Noise Cancellation through the menu options when you have the dialpad open hit menu options and then select something like call barring to bring up the "Real" Phone options menu and then you can turn off Noise Cancellaction to see if that corrects your mic issue while in a call.)

After I receive my new (or Refurbished) TP2 in a few days i will let you know whether or not it was a handset issue.

I was at first experiencing these issues on the Jun 1 29020 Sencity RHOD CHT 2.0 builds but I have the same issue even when after a couple Task 29 and running on the Stock Sprint MR1 Build - which is why I was almost certain it was a handset issue.

(If you have TEP, just task 29 and take your phone in...much easier than replicating the no mic issue)

P.S.- Experiecend issue on both 2.32.00WF and 2.32.00WU with PRL 51079

(Was sometimes getting the same issue on my EVO and EVO shift but it was only when the phone was 'mis-handling' when switching between Roaming.....but the Arrive hasn't given me these issues at all. (These 3 are running PRL 60679)