The only way to answer your question without any advertisement of any product on the market today: – Try all of them first and then decide, look at the basic features, functionality, quality and cost, make a lot of calls collect data. You should be very clear on what you need to setup your VoIP network.
I would disagree with gentlemen who said: “What you paid is what you get” this is only true when you advertise your own overprized product.