I replaced a screen on a iPhone 5 and had the same error. I pulled the screen back off and removed and reset the display cables and it went away. I have checked with my customer at least 4 times since the repair and she has not had any problems. At the worst replace the screen assembly. If a repair technician replaced the screen and it is acting like this why isn't he fixing it under warranty?
I have repaired two iPhone 6 units hear with the same problem. The ear speaker has small connectors that touch a ribbon type cable. It is possible one of the screws on the shield backed out and the connection isn't stable. To open the iPhone 6 is quite easy if you utilize a iSesamo or iSclack from iFixit. You would just need a pentalobe screwdriver as well as a Phillips and a nylon spudger. Follow the iFixit guide on replacing the digital display on the iPhone 6 and when you have removed the battery cable shield and disconnected the battery and have moved the emi shield and have disconnected the display cables you will be able to access the shield covering the ear speaker. It has 3 screws holding it in, they are Phillips type, check first to see if they are snug(tight) if not that could be the issue. Just follow the iFixit guide from there. If it still isn't working please contact me, I will gladly help via online if I can.