Work is there. People are paying to get VoIP installed. You do not sell VoIP. What is it that you are infact doing if not giving up work? If you choose to make that decision then more power to you. To each his own.

I don't have the luxury of voluntarily giving up work therefore I will try to provide the best service I can to a customer. If what the customer wants it not the best fit I will dutifully try to explain to them why it is not a good solution and stress that it might not work. If they still refuse to listen to reason then let them know you will install it at their own peril. You dont guarantee it to function correctly and all call-backs are billable (within reason).

We are paid to consult the customer and install what they want. We inform them of their options and let them decide on their course of action. If you tell a customer that if you install something they will be hit in the head everyday with a 2x4 with protruding nail, and stress that fact, and they still pay you to install it, then install the damn thing. You cant fix people's stupidity or stubbornness.

VoIP is not stealing work it is another option. Granted it's an option that is new and you will have to learn. 25 years of TDM phone systems wont prepare you much for VoIP if you've never looked at the computer network side of things.