The Cherry Mobile D23 all of the sudden the mic doesn't work. The Caller can't hear my voice but I hear them well on the ear piece. When I use a headset it works fine. It must be the mic on the handset itself. Please help how can I troubleshoot.

You have made it clear, your problem is with the mic. If you know how to replace it you can give a try, otherwise i would recommend you to take it to the repairing shop and get it repaired. It's will cost a maximum of $10 (rate varies with countries) including service charge. Universal Mic is the best, so if you buy one, ask for a universal mic. Good luck.