I worked for three different telephone companies for 33 Yrs, we used lead acid battery to operate all of our office, the switching equipment that did all the work, was fed directly off of the several different battery strings in each office. We used A.C. rectifier to charge the batteries and used the batteries to act as noise filters from the local commercial A.C. network as well as run the offices 24/7 365.
I took some training in Palm Springs, CA in the summer of 1965, the training room we used was in the basement of the then new Palm Springs east Central office, it had batteries built and used in the main office before automatic switching equipment was invented and used that ATT had installed in 1905 in the original and only telephone office in Palm Springs back when telephone operators handled all calls
Each of those cells where around 18 inches to 24 inches square and around 2.5 to 3.0 feet tall incased in either clear plastic containers or glass
All of the lead acid batteries are two volts and the national telephone standard operating voltages in the central offices varies from 48 VDC to 52 VDC, depending on the company and area of the country and in the World
Every string was supposed to have a monthly inspection and preventive maintenance(cleaning and adding water) performed on them to prevent them from going bad, most offices had that done by one employee. I saw one remote office that had been neglected by the tech , he would sign off the P.M. work that he didn't do, which ended up getting him fired