Square type fire alarm system handset with PTT switch A23

As a telephone handset for fire alarm communication system, how to solve the connection stable and reduce the noise from background? For outdoor environment, UL approved ABS material and Lexan anti-UV PC material are available for different usages; With different kinds of speakers and microphones, the handsets could be matched with various motherboard to reach high sensitivity or noise reducing functions; hearing-aid speaker could also be chose for hearing-impaired person and noise reducing microphone could cancel the noise from background when answering calls; With push-to-talk switch, it could improve the voice quality when release the switch.