Why does Android disable HSDPA/EDGE communication while calling?
Most HSDPA / EDGE CDMA networks don't support data and voice at the same time.  CDMA Modern GSM networks (including 3G and HSDPA, but not EDGE) do, and LTE will, so I'm not sure why you're having this problem.

It's possible that Dual-Transfer Mode may not be enabled on your phone.  Alternatively, your network might not support it.  (Info on DTM.)