Why does screen go black after dialing a number, preventing me from punching in extension?

Is it in 'tone' mode or "pulse" mode

Sounds like the proximity sensor. The phone has a sensor that attempts to determine if the phone is up to your ear, and then shuts the screen when it is to save battery. However, if you have a screen protector of any sort it can block the sensor, making the phone think it is up to your ear once you place the call.

Easiest way to test this is to disable the proximity sensor then place a call. If the screen does not instantly go black after you place the call, then you know for sure something is blocking it. If you have a screen protector, try removing that.

To disable the sensor:

1. Goto settings
2. Call settings
3. Additional settings
4. Uncheck the proximity sensor box

I’m happy to assist further over the phone at https://www.6ya.com/expert/christopher_d13aca62d29ed9ae

You need to disable the proximity sensor. The use of the proximity sensor is to detect if the phone is up to your ear during a call, to turn the screen off. However if there is debris on the sensor or even a screen protector over it, it constantly thinks it is up to your ear, hence blacking out the second you make a phone call.

You can go into settings and call settings to disable the proximity sensor.

I’m happy to assist further over the phone at https://www.6ya.com/expert/christopher_d13aca62d29ed9ae

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Try taking out the battery, then taking out the SIM card and blowing on the simcard's contacts
Then reboot your phone

Yes. I have to warn you first that some carriers will not be happy about this though.

First, look at your dial pad, punch in "*#06#" Star Pound Zero Six Pound.

The number that it gives you, your IMEI number, is what you will use later, copy that down.
Make sure it is correct!

Now, once you have that, you can go into your browser and type in "cellunlocker.net"
Go into the portion that says "Code Lookup", and when it gives you the make menu, select "HTC"

Now punch in your IMEI code, and hit the Search For Code button.

Now just punch in the unlock code it gives you and you're done! :D

Sounds like a video card problem
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There is a LCD strip (Bunch of closely punched wires, that makes up a connector). If you have replaced the LCD yourself them there may be chances that the strip is not properly punched. That strip comes out of display and goes straight onto the main circuit board.
This is a common issue that makes the Phone works perfectly but in a black-out mode.
I hope you have a clear picture now.

Regards, Waqar. If I was helpful, Kindly do Rate!

Flip open the one touch panel, Punch "Data Registration" arrow right to #3 Fax Settings, punch set, arrow right to Report settings, punch set, arrow right to RX Settings, punch set, Arrow right to OUTPUT NO, punch set. punch stop, your done!

You can register your Punch! product online at:
http://punch.custhelp.com/cgi-bin/punch.cfg/php/enduser/acct_new.php