So you're having a phone call with someone but they're particularly quiet and it's not a bad connection either, it's your phone. Here's how to increase in-call volume in iOS, Android and Windows Phone. See also: Best new phones coming in 2015.
It can be pretty frustrating trying to have a phone call with someone and you can only just catch what they’re saying if you try really hard and put your finger in your other ear. A lot of the time you're probably better off just having a conversation over text message. Increasing the call volume on your phone is really simple, though and should solve this problem.

How to increase in-call volume in iOS, Android and Windows Phone

You've probably been searching through your settings menu to find some kind of in-call control or volume slider but you won't have found one because there simply isn't one. You will find all kinds of other sound settings such as alarm volume, notification volume and more.
It might seem odd but the real answer to your question is that you can't adjust in-call volume until you're connected on a phone call. When you are, no matter whether you're using iOS, Android, Windows Phone or something else, you simply need to use the volume buttons/rocker on the side of your device to increase or decrease the volume.
If you're already aware of this and you're call volume is at maximum then you've got a different problem. Reviewing so many phones, as we do, we've notices that some earpieces dramatically change in volume with only a small movement while holding it to your ear – so you might just need to take some time to find that 'sweet spot'.
Once you've tried the above and the problem is still ongoing, you might need to try an app. Search your respective app store for a volume booster and see which one works for you.Volume Booster is popular on Android, for example.