Why can't anyone hear me when I call them or make a snapchat is there any help?

Hey guys so I am having this little issue with my IPhone 6 which I just bought. So ok the problem is that when I make a call or get on snapchat and make a video can't nobody hear me or anything and that's a BIG!! issue for me because I just got the phone. So if there anyone to help me please do cause I really need this problem fix asap !! Please and Thankyou.