Don't be surprised if the EMA is tight lipped about it.
The radios are typically are part of the control box on the modern sirens. The control electronics and the receiver are usually in one box, and the batteries are in another for the sirens that use them (2001DCs, Mods etc) because charging batteries can generate gases that will eat up metal.
The classic Thunderbolts typically had two cabinets, one contained the RCM1 motor starter (which is the relay box that switches the 3 motors in the Thunderbolt on and off as appropriate) and the other typically had a Federal AR timer and a Federal ARCH radio receiver on whatever frequency the controlling agency used. The other cabinet could also have a telephone relay setup if it were one of the systems that was controlled by leased lines in place of the ARCH receiver.
I've personally never seen one of the landline based setups so I'm not exactly sure how that is interfaced to the AR timer.
Our own Thunderbolt master CDV-777 has some excellent pics of the guts of the RCM boxes and the AR timers if you want to see them
here
I highly recommend looking at his whole Siren restoration set of pages for your project if you haven't already.