I find the life expectancy of any phone system as a whole is equal to the life of the power-supply. Phones wear out and some brands last longer than others and get replaced. 10 years is a long time for something to be powered up 24/7. I tell my customers it’s like a TV that never gets turned off. They relate to that. Most manufactures that I deal with engineer semi-conductor tolerances into their product. So after 2 years under constant voltage when a semi-conductor breaks down the rest of the circuit is designed for that and compensates.