I had a phone system for a small office fail a few years back. We called a service company out and one of the first things he checked was a supercapacitor on on

I had a phone system for a small office fail a few years back. We called a service company out and one of the first things he checked was a supercapacitor on one of the processing cards in the system that keeps some volatile memory stored through a power outage. Sure enough it had failed and in a few minutes he had it desoldered and replaced with a new supercap, he already had plenty on hand in the truck. I figured at the very least they would have swapped the card as a whole and maybe refurbish the failed one later on but due to the common failure of that supercapacitor they just always replaced it onsite. If that phone system had been designed to maintain that memory with a lithium primary battery instead it could have even been a user serviceable component.