These are all great solutions when you are considering a PBX for a single company that may have multiple locations, remote workers or even a company that has multiple brands. The one thing these all share is the fact that all the users are together. Sure you can use some CoS or other items to help control what those users can do but there is generally no worries about “cross-contamination” because at the end of the day, they are all part of the same corp.
In a “Hosted Voice” a.k.a Mult-Tenant solution that places like RingCentral, Jive, Vonage and basically any Telecom out there that offers this type of solution the one single expectation that exists for this is that the “tenant” is private. That means the client basically has an PBX to themselves and the freedom to do as they wish with it. This is like any other established voice service out there when you say it is “X type of service” then there are going to be Y expectations for that service.
The OP has already said “I don’t care about the same extension for 2 or 3 companies” but you know who will care? The company that has existing extensions that are used by the public and in their contact info on their material/emails/business cards. Now for some companies having to modify/change their extensions won’t be a big deal because in those cases the extensions aren’t really “public”. However, companies that have known extensions that can be direct dialed by the outside world will have issue with having to change everything and just will opt to go someplace else that doesn’t make them change their entire setup.
Also the other fun caveat as everyone as dismissed incoming calls as “easy peasey”. What happens when Company A wants to have Direct Dial enabled on their IVR? Because if you don’t set up the Direct Dial properly guess what happens? The caller can enter any extension and if that extension exists they will be able to call it. So again, they could end up calling Company A, getting their IVR and then doing a direct dial to an extension that used to exist for the company but now has changed and they end up calling another user at another company. Then there is the whole “mis-dialed” issue that callers have and they could mean to enter the proper extension and end up entering the wrong one, it could then just end up connecting to another user in another company.
There is a lot more to making this a “Multi-Tenant” system than just throwing modules, free or commercial, at it.