I am looking at setting up an Asterisk PBX for a large student housing project. We will have 1 extention for each suite of 4 students, and we will adding aproximatly 20-60 suites a month untill the project of 1200 suites is completed . We will also have about 100 extentions in offices.

All outside call will be handled by a provider who is charging us a flat rate per line (1 line needed for each concurrent call). We would like to have the minimum number of lines needed, but we also want everyone to be able to get an open line when they want one.

We need to come up with a reasonable formula for the ratio of extentions to lines. We can see that a 1:1 ratio would be overkill, but a 10:1 ratio is insufficient. Can anyone with some experience give us a little guidence?