The next time someone calls you, use the volume up button to increase the volume of sound you hear in the earpiece. This only works when you're in a phone call but the setting will remain where you move it to until you change it again.
If a visit to an Apple Store doesn't help, you may want to consider having your hearing checked, just in case you're having trouble hearing the sound frequencies put out by the iPhone earpiece.
It's entirely possible that the earpiece speaker on your iPhone puts out different sound frequencies than the one on your old Nokia phone, explaining the difference in perceived sound quality.
You're welcome. I misunderstood when you said you "cannot hear very well when people ring me." I thought you meant the volume of the ringer. Good luck.