You could at least mention the iPhone model you have so we can give you specific info. Not to mention you could have googled it and not have to wait for someone to answer...
Check these link: http://smallbusiness.chron.com/change-ear-volume-phone-calls-iphone-31569.html
Thank You . I was talking about iPhone 6 but the problem is with all the models and the link you have given is a general case explains the general tips - sometimes 100% of the volume is not audible.