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Hi @sbfawkes, welcome to our forum!
I'm sorry to hear about that. 
The screen should be turned on when you take the phone down from your ear because the proximity sensor no is longer covered, is that working correctly for you or is the screen still turned off and that is why you can't use the dial pad if you for example, needs to enter "1#" to make a choice in the call to talk to another department or similar?
Or is the screen turned on but the phone is locked with your screen lock and you can't unlock it during the call?
I'd recommend that you try rebooting your phone in safe mode to make sure that a downloaded application isn't the cause of this.
I'm sorry if I misunderstood your question!
Thanks for the swift reply! 
There is a sensor called proximity sensor that is placed at the top part of your phone, just to the left of the Sony logo (number 4 on the picture below). When this sensor is covered by something during a call for example your ear, the screen will be turned off. This is to make sure that you accidentally do not end the call with your chin, and when taking the phone down from the ear the screen will be turned on.
Was there any change when the phone was rebooted in safe mode?
If you experienced the same behaviour in safe mode, do you have a screen protector that is covering this sensor, or is there any dirt/moisture on the sensor? If there is, try wiping it clean with a cloth and run the diagnostics test in the phone, which you can find in Settings > About phone > Diagnostics > Test > Ear proximity.
Let me know how it goes!
Thanks for getting back to me!
If it works in safe mode, make sure that all your applications are updated to the latest version, you might have an application installed that needs to be updated so it's compatible with the latest software version.
Let me know how it works after you reactivated the theme or if you found out which app is the cause!
even in safe mode when I move the phone away from my ear the screen stay locked. Proximity sensor stays On for no reason. I don't have screen protector at allHi @IvanBG,
Sorry to hear about that. 
I can see that you have an Xperia Z3 registered on your account, is that the phone you are experiencing this behaviour with?
Since it's not working in safe mode, I would recommend that you perform a software repair using Xperia Companion.
This will delete all the data stored on your internal storage, so creating a backup using Xperia Companion before starting with the repair is important.
If there is no change after the software repair has been completed, your phone will need to be examined by a trained technician at an authorized service centre.
Your Local support team will assist you with this and answer all questions that you may have regarding warranty, service routines, consumer laws and so forth.
Keep me posted!