Hi,
The problem is definitely connected to the proximity sensor.
Two common problems occur after/during a screen replacement.
1. The distance between the proximity sensor en the new screen must be 'none' because any light leaking onto the proximity sensor* will give a false positive and your screen will go blank as it should when you put it to your ear. This happens on all android phones, basic setting is screen off, as with the iPhone's the basis setting keeps the screen on. What’s better is another discussion ;))
2. As Recardozo90 said, some android phones, like LG, Samsung, etc. have a small rubber piece, keeping the proximity sensor in place, and pressed to the screen.
You can check the first by pressing a little on the top off the screen, pushing it down into the housing. gently, you don't want to crack your screen. If the screen has the slightest room to wiggle up and down, the screen isn’t placed properly into the housing. If so press the screen down and dial your voicemail, the screen will probably stay on (unless you covered the sensor) and will go blank as soon as the screen lifts a little from the housing.
The second is pretty easy, as the old housing/frame of the old screen will probably still hold the rubber piece.
With regards, Brenda
* the proximity sensor has an infrared light, right next to the sensor, that sends out a beam and if reflected on to the sensor it detects your face/ear and the screen goes blank. If the beam reflect on the back off your new screen, the sensor gets a false positive