i think the OP is talking about the screen turning off during a call when using a bluetooth headset. No proximity sensor involved then. I haven't seen an app to keep the screen on during a call (using a headset or not) so like was pointed out previously, set the screen to never turn off when plugged in (in the development category under settings) or set your screen timeout to never, of course this way you need to remember to set it back when you unplug from power of you will drain your battery fast.
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