I want to know what requirements would I need for 15 to 20 phones. I don't need any outside service just station to station on the vessel. I'm fairly new to VOIP. I've found tons of information on standard systems. I was wondering if building a system that doesn't need to reach the outside world and doesn't require a VOIP service requires the same hardware/software. Any information would be very helpful. Also if you have any suggestions for low cost systems that would be great.