As someone with an over the top voip setup. I would question why anyone would want a VM telephony solution.
To put my system into perspective. I have 2 Sipgate basic circuits. Plus a 3 line Sipgate trunk that costs £5 per month. The basics cost nothing. Then there are of course outgoing call costs.
I connect all to a pi running freepbx. I work from home so one basic and a trunk line are assigned for work calls and fax. A second basic is assigned for the house.i have sip phone in the office a second in the house. All household mobiles also run SIP.
Meaning anyone calling the house get ivr directing to my wife or I. Even when mobile we still get the calls etc. To the office the ivr offers sales service development etc.
The best part was at one time I had numbers in three local towns. Meaning for a small business, it made me look bigger as customers would prefer to call a local number and not an anonymous mobile etc.
Also far easier to call between office and house and to be able to transfer the calls etc.
All for the cost of a pi