These devices seem to operate more like simple walkie-talkies rather than adjustable ham radios/complex two-way transceivers, in spite of their appearance. They don't have a frequency dial, nor do they display any frequency/channel information on them anywhere. The fact that characters can broadcast messages to all other callsigns seems to imply some shared frequency, though, with perhaps some kind of alternate frequency channels or encryption method to make the private message system work. Either way, the only broadcasts Senku will receive will be the messages from other characters in public conversations.