proximity sensor of White iPhone 4 was able to transmit signal but was not able to receive them properly. It is being fixed now by iPhone Repairs Sydney Specialist.
Increasing numbers of iPhone 4 users are reporting the touchscreen becoming active during phone calls when the proximity sensor would normally shut it off, leading to inadvertent button-presses. The reports are coming in with enough frequency and consistency to suspect a hardware or calibration issue with at least a particular subset of iPhones. No word from Apple yet.
The iPhone's proximity sensor uses a small infrared LED and uses the reflection from nearby objects to deactivate the touchscreen when it's close to your head. If you hold it up to an iSight or other camera that can read near infrared light, you can usually just see the glow. The problem may be in how sensitive the detector is set to be on some phones or an actual problem with the detector.
Only iPhone specialist can show this successful real video which shows Proximity Sensor works well on White iPhone 4.
If you don't get this result after white iPhone 4 conversion service from any iPhone Repairer or white iPhone 4 kit seller, You must ask them to offer a full refund.