So what exactly did you put in which dial string to accomplish this? Also, can it repeat dial if the phone continues to remain off the hook with no connection? In order to be an effective approach, it needs to continue until the phone is put back on the hook.
I have also been looking at the phone line polarity. Under "Physical Interfaces" -> "Phone1 Port" -> "Tip-Ring Voltage Polarity" there is a section that might make it possible to detect the status of a call by the polarity of the Tip/Ring wires.
In my case, it would be easier if I could get your approach to work because I would only need a single circuit, say an ESP wifi board to emulate a SIP server, plus the amplifier. This is because the PA system is in a central location that does not have access to a phone line. So if I use the polarity method, I have to have two units, one to detect the off-hook condition and another to activate the PA system - connected by a wireless link.