I have only done this one time, because once you send a call to the paging system it won't go to voicemail. You should probably use a night key or schedule to turn on an Auto Attendant at night.

It has been a while, but if I remember correctly then you want to setup a page group with the external speakers, and then DIT dialing digits to the group paging code, ie.. #3101.

On some telephone paging amps they have a contact for a night ring. If you used DIT to night ring, then the relays on the processor with clost to trigger the ringing sound from the amp.