On-Premise VOIP PBX is less expensive than the hosted service in the long run but, of course, has a larger up-front cost. The crossover where it becomes less expensive to have the on-premise system is sometime in the second year of ownership. Companies with more than 20 users will probably find the on-premise system the best way to go.
Note that “on-premise” does not necessarily mean that the server has to be in your office. We have customers who put their server in a hosting center so that they have the same power and connectivity redundancy that the hosted service has while owning their own system.
The On-Premise system can connect to any type of existing or future phone service you might have including analog lines, PRI and SIP trunks. SIP trunks provide extremely competitive rates and the phone service comes in over your Internet connection so there is no hardware needed to connect to them.