At the bottom of every Symbian's Phone Features:

"Variation in operation times will occur depending on SIM card, network settings and usage. Talk time is increased by up to 30% if half rate is active and reduced by approximately 5% if enhanced full rate is active."

How do you activate half-rate/full-rate on a Symbian phone?

If I remember correctly, you type *3370# or #3370# on a Series 40 phone ...