I had the same problems with unintentionally muting/ending/speakerphoning my conversations but I may have covered up the proximity sensor with my adhesive screen protector. I've read if you remove the protector, cut the new one slightly smaller so it doesn't cover the sensors, it might help the sensors work correctly.
Also, I followed the "download the Sensor List app from the 'Market' " instructions listed by Seraph under the discussion titled: Proximity Sensor - is sprint aware of the issue? Help! posted by Maryinsouthie and my Samsung Moment was stuck on "Waiting for value update" under the GP2AP (which is the label for Proximity Sensor) and it stayed stuck on "waiting"...
I then chatted with a Sprint Chat Rep and she asked me to power down my phone and take the battery out so she could then "Update my phone". Then she asked me to turn my phone back and try the Sensor List app again and sure enough, the update she did on my phone worked because I am now able to see the values for the Proximity Sensor (if yours works, you will see green, red and blue graph-like lines and numbers). The moving lines and numbers make you aware of if your sensors are actually working because if they're not, your phone mightneed to be replaced.
I suggest anyone having the same problem with the Sensor List app log online and ask a Sprint Chat Rep to update your phone.
Good luck everyone!
I have had the same problem over and over again - I'll be talking to someone and it goes to speaker or it goes to MUTE or it simply hangs up!; also have problem with the batter going dead; got the update - (by the way, if you have any free apps, after the update, you lose them!!) - Sprint offered to send me another battery - i put the new one in, still doesn't work well. Guess what? I bought the phone in Jan; started having problmes right away - last month (April) was when I got a replacement phone but sent it back - I paid a lot of money for the NEW phone - they send you a REFURBISHED phone! Watch out! I also have problems with the phone locking up when dialing! It will continue to dial and you can't hang up, you can't turn your phone off or anything! you have to take the battery out to turn it off and it is hard to take the cover off to remove the battery! I am ready to toss this phoneI This is the worst phone I ever had! It has given me a signal that I was in airport mode a couple of times too.....