The A640 Samsung phone sends at very low volume. To the person on the receiving end, it sounds as if the person speaking into the Samsung phone is far away from the phone even when that person is speaking directly into the Samsung A640. This happens even when the battery is fully charged.

The phone is not so likely sending at low volume as receiving at a low volume and transferring the info that way. Your microphone may be either dirty or damaged. Check to see if there is any lint or dirt in the tiny microphone holes, or visible damage due to dropping the phone. Shut the phone off and remove the battery and sim card if accessible. Try vacuuming with a soft brush. Then reassemble and check your volume settings and try a new call

I had a similar scenario with my A920 . I could hear people answering but they could not hear my voice at all. It turned out it was caused by a defect in the circuit board although in my case it was also related to the charging system as well.