A hosted PBX solution may not be right for every business application. But if you do your due diligence you may find it's a good business choice for you.
One key is that when you talk about hosted PBX (VoIP) .... it can mean different things to different people.
The intent of hosted PBX / VOIP is to provide the service at a fixed monthly cost rather than taking in the capital cost of a purchased solution. It can also be used to invest in a more feature rich solution than would otherwise be affordable (renting a piece of a very expensive system vs buying a lesser system)
There are two types of hosted providers out there - on premise and off premise. On premise providers bring in the equipment to YOUR site and install it - you just pay for it monthly (think fully managed lease).
Off site providers use private WAN or Internet connections to provide phone services for your organization.
One of the biggest problems you will face with the number of users and simultaneous phone calls will be in the connection between you and the hosted datacenter. With say 200 - 300 phones .... and an average use of 10% for normal businesses .... that's 20-30 phone calls. Depending on the codec in use that's between 900 Kb/s and 2Mb/s. That would only cover 1-2 T1s of usage. Depending on the platform interoffice calls may also be routed via the hosted platform - so that should be a consideration as well. If cost savings is your primary concern - then this could be an issue (depending on the price of bandwidth in your area and whether its on premise or not).
Many organizations can see benefits from hosted including reduced capex, organizational efficiencies, productivity enhancement and much lower maintenance and management costs. However I would caution you to avoid platforms that do not provide QoS throughout the network. There's a big difference between the soho / smb product lines and an enterprise grade service; and QoS over the network (not just the LAN or at the edge) is one of the biggest. Additionally, you should have a consultant perform a LAN assessment to determine the ability of your LAN to handle the VoIP traffic without degradation. It may require upgrades, adding to the expense.
I'll be the first to say that this solution is NOT right for every customer application.
VoIP in all forms is viable depending on the needs of the customer and how it is engineered. There's a difference between enterprise grade and consumer/smb/soho offerings. I suggest you spend some time with a qualified and objective consultant. For a relatively small investment of time they can educate you and help guide you to a decision. For this I suggest you take advantage of the no cost assistance offered through Business VoIP Solution.