I done a few installations like this the biggest factors are
how much background noise
how big of area and obstructions
how many seperate areas to talkback from
how many locations need paging origination
what type of enviroment
feedback is only an issue if paging telephone is located to close to speakers

Merritt

Business Telephones & Equipment + Commercial Audio/Video Products
Commercial Communications . . . Turner, Maine
If it was built after 1980 don't expect it to work right.