I don't know about AC's FD having volunteers or recalling paid firefighters, nor am I sure what they're doing. I won't guess.
There are some misconceptions about trunking and alerting.
You CAN alert radios in most trunking formats. You can even alert in P25 conventional and it is FAST! Not 4 to 6 seconds (or like 30 seconds to send a list of stations), it's less than 1 second and can't even be heard over the air! Because many volunteer departments can't afford to buy their individual volunteers trunked radios, they generally retain their prior alerting methods. There is no such thing as an 800 MHz or trunked pager and those would be considered niche market products with a net loss on the engineering investment (that's why there's been little innovation in alerting since the Plectron days, and more advanced formats like Zetron's Model 6/26 FFSK haven't taken off). You CAN partition a trunked system to always put a certain ID or talkgroup on a certain frequency. So, non-7/800 MHz trunked systems may actually be able to alert page to Minitor pagers (or other receivers), although the page may be delayed or held in queue until the channel comes available if the system is busy.
I've seen two systems, one in South Carolina near Myrtle Beach, the other in Missouri, which are sending two-tone signaling to trunked control stations through analog trunked radio. After-market two-tone boards, like CommSpec, are installed on control stations in the fire station and will unmute the receiver and beep. I'm not sure how (or if) that would work on P25.
There is also a trend now to have TWO INDEPENDENT alerting methods, which conforms to the NFPA1221 standard. In that way, alerting may be done on trunked mode, and on VHF independently toward a reduced ISO rating (the lower the rating, the lower the cost of insurance in the community).