Ah, Nokia. One of my favourite phones was a Nokia 3210 once I had put the vibrating alert (a small motor with an eccentric weight) in. A nice shape and would fit in my shirt top pocket just right.
I remember those well and the 3310, infact I have a friend who works in a scrap yard and he still uses a 3210 at work
I had access to unlocking gear years ago when these phones first come out and you could add pics, screensavers, ringtones and other cool stuff using the unlocking cables.
I'm sure you could flash certain xx10 models to upgrade them to the xx15/xx20/xx30 handset firmware to add extra features to the phones