telephone is  a good and simple way to have direct intercomunication

amplification

for collective use telephone must have a loudspeaker system  -built , - or amplified with a public adress and a microphone sticked to the telephone or any other way

call cost- with voice on internet services

in europe it is cheap to call landlines in brasil  - for instance use this number 0811310310  then  after listening the message  dial the brasilian number in belem  005591 xxxxxxxxx  and finally dial # 
......and you can talk for long for cost of a local call

in USA ???

in other countries ?? you can use skype call to call a telephone from you computer

in belem

facilitating team of belem expanded is trying to secure rooms with a telephone that can be called from abroad  and amplication