Why is the volume on calls so quiet at maximum volume but ok on speakerphone and ringtones?
The best thing to do is bring back your phone to the store you purchased it from. Let them know that there is a possible issue with earpiece. Ask if you can compare your phone with another new Nokia 225.
If volume is weak even with the other unit, it only means that it's just the way it is. Just use a headset when taking phone calls.