I have a spare Android phone I keep on with a SIM card in so work can call me for server emergencies even when my regular phone is on silent. With 3G, Wifi and data access disabled, it lasts for over a week on standby, even with not-so-great reception.

If you're fine with returning to the feature set of that era, modern phones have huge batteries and highly optimized chipsets that exceed the performance of the old phones.