How to disable proximity sensor
Motorola CHARM has a proximity sensor located on the upper left corner; as stated in Motorola CHARM product support site, external lighting conditions will affect this sensor.



My suggestion is: each time you need to use the touchscreeen while making a call, try to "trick" the proximity sensor illuminating it with a flashlight or LED, so the sensor "thinks" that you've moved away the smartphone, thus re-enabling the touchscreen circuits. 

If you don't have any of these items on hand, look for a place with bright light toward which to move yourself/your phone.