When a call is sent or received from mobile phone, if the sound is very low to the other side person, check the micro phone (mic) hole area in the mobile phone which may be filled fully with dust. If so, clean that hole area with needle type object. Be careful that the needle should not be inserted deeply inside that mic hole which will damage the microphone permanently.
Most of the mobile phones will have micro phone hole in the bottom area. Some mobile phone pictures are shown above, look at them.
Omni directional microphones are used in all the mobile phones, where the sound will be received from all the directions. If the problem is still same, the micro phone of the mobile phone has to be replaced because of low sensing sound of microphone. Also there could be problem of micro phone circuit on the circuit board. Contact a qualified mobile phone technician.