Hey Lillybelle, (cute nic btw) unfortunately I do not have that phone but I did a couple of things to try and help you out, as so many here have been so wonderful to me even to my silly questions, so I did a couple little things for you to try and help you out. If you still find yourself in a bind, I would recommend either Creating your own thread on the forums here, though I do not believe that there is a dedicated thread for your phone, you could always post it where you think it should go and if you picked the wrong place, the mods would be kind enough to help move it to the appropriate place.
Ok first, I did a little research on google, as I personally have never even heard of this phone, though it may be distributed to different markets with different names (ie my late HTC Inspire 4G branded for sale in the US via AT&T, was actually physically identically to the HTC Desire HD which was slightly different for the EU market) but I digress, I found an nifty article on a site relating to this problem.
Not having heard of this phone, and the fact that I like arguing with the morons over a Samsungs Chat Support Rep's, I contacted them to find out what information I could about the proximity sensors. Now according to Samsung support rep, this phone does infact have the proximity sensors but they are supposed trigger automatically with out the use on an app or check box some where. Since you said that you were having this problem a little light buld showed up over my head, no wait, that was just my kitty! So I decided to use my best friend (well second best, cat's the first) and I found this following site
Free Samsung SGH-i857 DoubleTime (Samsung Habrok) DHD Proximity Sensor Recalibrator App Download in Display & Flashlight Tag so you are probably asking what does a flashlight app have to due with your hanging up on people or face dialing..... Ahhhhh, now here is where you should try and get Samsung to permanently fix this if they haven't already but we know how long it takes them and AT&T to release anything timely. Anyway, this guy wrote this flashlight app that gives you the ability to calibrate the phones proximity sensor. Give that a try lillybelle5668. I hope that works for you and or you have found another way to fix this issue with your device, as I have never even heard of the Samsung Double time, and when I asked them on their chat support this morning for you, with this phone there is no way to enable / disable, it should be auto matic. Best of luck, and I would never ever ask this because I'm not usually one to ask of something like this, but if this helps your problem could you please "Thank me?" I want everone here that helps me realize that I due try to help where I can, but I am still pretty much a n00b when it comes to the Android OS. I wonder if there is a Android for Dummies I could buy lol! All my best!