You can't send or receive email while on a call. If you are accessing data (i.e. website or email) and receive a phone call it will go to voicemail. It can appear as though you can surf and receive a call, however when you are surfing you arent always accessing data. If you receive a call during one of those periods when you aren't actively accessing data the call will go through.
This is a limitatation of the device. At this point there is nothing you can do to change it.