IP Centrex. Hosted PBX. Hosted VoIP. Virtual PBX. Call it what you want, but the important thing is to use terms that your customer base will understand.
It isn't about 100+ features. It isn't about on-premise versus off-premise. It's about the message that you are removing the technology problem for the business owner.
Hardly anyone outside our industry understands the term Hosted PBX. Even in our insudtry, it is called a number of different things. (Same with SIP trunk).
you have to realize what business you are in. It is not being a phone company. That's Ma Bell. Your business is in taking away the pain of the technology. At the end of the day, people just want the phone, the Internet, email, their car to just work. They don't care how it works - just that it does.
They would also like it to be easier to use (and the same as it was, please). As a service provider, can you take a way the actual technology -- offer them a blackbox if you will that just magically does what they need done.
It's not just how you market Hosted PBX but any managed service or SAAS.