Milk dried to a solid (casein, part of milk, was one of the first plastics discovered). Find the mic hole (if you don't have a manual, google
<your phone make and model - without the brackets> manual and you can find a picture of the phone with everything called out in a picture or two. The mic hole is on or near the bottom of the phone.)
Use a tiny
wooden toothpick and clean the hole out. Some rubbing alcohol, dripped into the hole with the toothpick may help to loosen the casein. Then it
might work again. If the milk got inside the phone ... see
Oh, no! My Phone got Wet! (It's probably too late to fix it now - it's been over 4 hours since you posted.)