Do you have long and curly hair? 
Itās not. The proximity sensor is a separate sensor that doesnāt work by detecting (visible) light like a camera does. Read more about proximity sensors here: Proximity sensor - Wikipedia
No My hair is short(ish)
I have been in touch with the support team regarding my problem with interrupted calls and they adviced me to try this app
https://play.google.com/store/apps/details?id=com.duckbonecallguard&hl=en
I installed it yesterday and I hope it will solve my problem.
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same problem here with additional issues:
when I use my right ear, things go well (screen goes dark when I put it to my ear, no extraneous actions)
when I use my left ear, things go crazy: after a short phonecall I find that the system brightness has been changed, a new calendar entry has been created, and various other actions have been performed.
Plus, with either ear, the screen stays black for anywhere from 1-30 seconds. Sometimes I have to press the power button, then enter my passcode in order to hang up or enter info into a touch tone service.
So clearly the sensor is not working correctly, and not only that, but it seems to prefer my right ear. I have piercings on my right ear that make it inconvenient to use that ear for phone calls. Never had such issues with my old iPhones (3G and 3GS)- for those if the screen stayed dark (rare), a little shake would wake it up instantly.
PS I have long hair, but itās usually tied back or tucked behind my ears.
I also forgot to mention that with apps like skype or viber, sometimes my ear hangs up 
The current configuration of the near field sensor ( @Chris_R, maybe you could change the topic to be more clearly that this is about, so others who have no idea what that sensor is are able to find it?) is a problem for me as well.
I often find myself in Time Zone settings or Calendar when finishing calls and i very often hang up by accident.
Interestingly, I had the same problem with other phones like the Nokia N8 or a cheaper Acer Android Smartphone (which is powered by an chipset of the same manufacturer as the Fairphone).
So tuning the Fairphone FP1 sensors would be great. A better sensor is on of the top priorities of my personal FP2 wishlist.
I have, for example, never had problems with the near field sensor.
So it can well be that it is a specific way of holding the FP (or other phones) during the call, which is causing the problems.
Workaround is to deactivate the option End call with power on/off.
While this only helps in the specific case were we end calls while the screen is off when trying to turn it on, thank you. All the other problems a probably not affected by this workaround.
Anyway, were could we find that setting?
Sorry, I had posted this in another post and was too lazy to write it againā¦ 
I searched for the fairphone chipset and proximity sensor and yes, there are other phones with the same problem. But more interesting is that in at least some cases there are solutions! The trick is to trigger the calibration routine of the chipset. This can be done in varous ways. There is an app that can do it but the problem (at least on some phones) is that it must be done every time the phone boots so the app is probably no long term sollution. āRusselSā has posted a solution to another phone that uses a script. Sounds that the best and easiest solution. The question is if this works for Fairphone as well. To test I must know how to run a script in Fairphone at startup. Anybody knows?
Here is the solution described:
http://www.modaco.com/topic/363179-proximity-sensor-problem-solved/ (post #15)
And here is the app:
https://play.google.com/store/apps/details?id=cn.zte.ProximeitySensorClib (The app claims that you can uninstall afterwards. Testing will tell if this is true for Fairphone and if it at all works).
Second best solution (so far and if this fails) is the call screen lock app described above.
I will try the app first and if it works try the script.
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I agree that it is not a light sensor. However I canāt confirm that it works only with conductive surfaces. e.g. it also workes with fabric, wood, etc. (Iāve tried that with the sensor display of SatStat).
Still I also have similar issues as described above. I think the delay is not (only) caused by an insensitive or too sensitive proximity sensor: After all it works quite well when I fiddle around with it in other apps like SatStat.
@Bengt and @jftr great problem solving spirit. If that really works, especially the init.d stuff, maybe it is even possible to integrate this in FP OS sometime!
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OK, the sollution do not work for Fairphone. The script runs a calibration routine made for another ROM (phone) that is not included on Fairphone. But there is probably something similar on the Fairphone so maybe there is a solution but it must be directed to by the Fairphone development team.
To check the proximity sensor (and others), test this app:
https://play.google.com/store/apps/details?id=com.mtorres.phonetester
To clarify things: Is there a ways to trigger the calibration of the proximity sensor on the Fairphone? If so, this problem can probably be solved.
I flagged this up to the FP team and they said they are looking into it.
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Continuing the discussion from Screen protector folie:
@jpjacobs asked this question in the screen protector thread.
Does the proximity sensor need the opening on the screen protector?
Could it make a difference?
What is the screen protector configuration of those who have had issues or at least some inconsistencies with the proximity sensor?
I too am using a 3Mās Vikuiti DQCT130 screen protector that although fits perfectly on the Fairphoneās screen, does not have an opening for the proximity sensor.
I am inclined to think it shouldnāt matter because I canāt seem to notice openings on screen protectors for other phones, but I certainly donāt know for sure.
What do you guys think?
I actually have problems with the proximity sensor and i have screen protector. Never thought there might be connection, but who know. Sad i would have to damage my protector to verify.
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I too use a screen protection but I would be very surprised if that have any effect on the sensor. The proximity sensor reacts on conductive materials and the screen protection is not that.
I have realised that for long calls, more than 5 minutes, I had 90-100% drop. Yes, almost every time the call was dropped. Most strange behaviour and that I cannot understand or even relate to any proximity sensor problem is that my calls are never dropped for short calls. (During good net coverage). That means that the drop occurs only after some time.
I have now started to use the Screen lock app and after that not a single call has been dropped! Once I noticed a strange behaviour that point to the proximity sensor: During a call I saw that the screen suddenly lighted up for a short while. Just a second or so. But because of the sceen lock app, nothing happened.