The mobile question itself is a moot point for me, since we provide both proactive and reactive 24/7 services to all of our customers.
Everyone in my company needs to have a cell phone, so we pay for it. By turning these into our standard method of communications, we save money. What would be most ideal is if we could find a way to make all of the phones spoof their caller-id as being our company's 888 #
The problem with your of a phone system being $67/month is that we're both consultants, but of different sorts. The main difference is you charge your customers T&M. My model is more of an HMO model where my customers pay a high premium, but we own the risk for anything that goes wrong or needs doing.
Internalizing and maintaining knowledge is expensive. My minimum customer opportunity is $30k/year mainly because of the complexity of proper, accountable knowledge management. This sort of knowledge, maintaining an immature VOIP PBX, has no profit in it for me.
I'll gladly pay "extra" for somebody else to do it, for not having to bear the risk of hosting the infrastructure in my office or datacenter, and for not having to maintain any in-house knowledge of the system.