To answer your question without bias, you essentially enter the telecommunications systems trade just like you do with any other. Electrical and plumbing have structured licensing programs based upon local laws. With telecommunications, it's based upon the same standard. Unfortunately, there are no laws enforced by local authorities, so anyone can go around doing whatever they want without repercussion. The system manufacturers are the only enforcement bodies actually verifying the knowledge level and financial responsibility requirements that a regular trade license would require. That means that they can cut a contractor off at any time if the contractor isn't doing a good enough job supporting the customers or coming running quickly enough.
It basically comes down to the school of hard knocks, just like getting an electrician's license. Plan on spending about four years of OJT to get a good understanding to the extent that you can be left alone on an installation. Another four years will be required before you have the verifiable knowledge of the industry before a system manufacturer will acknowledge you as an authorized dealer.
As I originally mentioned, it's probably best to partner with a local trained and authorized dealer. This is the very same thing that you would probably do with any other trade, such as plumbing, carpentry or HVAC. Although the installations of phone systems appear to be fairly new and easy to learn, you won't be doing yourself or your customer any favors in making assumptions. Let the dealer pay you a sales lead for the equipment sale, you do the wiring, and then they can deal with the emergency "how do I do this?" questions in the middle of the night. Such a relationship will greatly reduce the headache factor.
As HBiss mentioned, when the phones don't work properly, for any reason including user-error, it's a total crisis. Can you afford to drop what you are doing on a major job to come running only to find that they have the phone plugged in the wrong jack? Trust me, it's not worth it unless you have a lot of people working for you who know what they are doing so that someone can be there within minutes, not "later or tomorrow". I've been dealing with these crises since 1983. There's way much more to a phone system than meets the eye.
Don't forget, the phone company will completely throw you under the bus if they find that the problem is not in their facilities. You'll need to be able to prove them wrong.
Now, as to that monthly maintenance contract fee that was mentioned by others, I'm at a bit of a loss on that. That must be something new in NJ because we would never get away with such a thing around here.
