These days a phone, and some means of recharging it in real time (hub dynamo, booster battery) even on day rides if it provides other services beyond emergency calls, are just about essential bicycle equipment.
I have (so far) resisted getting a smartphone, so with limited use (on all the time during waking hours, a 10-20 minute call home every day) my basic ten-year old mobile phone will easily last a week on one battery. I do generally carry a second battery, rarely the mains charger.
