After working well for a couple of months my Lumia developed an issue whereby every time I made a call the screen would blank and subsequently refuse to return to normal operation.
After a little searching I discovered ‘the fix’ to the problem which occurs due to dust build up around the sensor:
Simply blast some air* all around the top area of the phone and the problem will go away!
* I used a good old fashioned blast of air from my mouth but if you have a can of compressed air handy that is probably a better option.
NB If you had this issue and this helped you please leave a comment below!