We only have a landline because of our cable package and we have NOMOROBO set up to avoid spam calls. It is very rarely, if ever, used.
When we got android phones I deferred to my kids for advice. Here is what they said: Close all apps, leave on charger when not in use. We don't have iPhones but it does turn on when plugged into the charger. We've had these phones for probably about 5+ years and they still behave like new. We also keep them on 24/7 because we have elderly parents who may need us in an emergency.
Someone above mentioned draining the battery by inefficient charging. I was once told to drain the battery before recharging fully, but that made the battery wear out faster. So now I keep things on their chargers and it seems to save wear and tear on battery life.
I don't know how long phone batteries are supposed to last, but my philosophy is when the battery dies, get a new device. It's just not worth the expense to replace batteries in old technology since there's always something new and better on the market.