I thought that fire trucks had a switch to activate the sirens by the drivers; I had no idea they were activated remotely by radio.

I was referring to the standalone towers in the Midwest. Maybe elsewhere. When I was a kid i would wake up on Saturdays at noon due to the siren. My dad also had a police scanner I could hear. One summer I put 1 and 1 together

All you need is a dtmf decoder and a small transmitter to set the off those large sirens on high poles throughout that sprinkle the country in small towns and probably large cities