Background: When I started my life path I learned the Golden Rule: He who has the gold makes the rules. This a bit translated is good customer service. Customers say and they should receive or you can leave.
So in the case of no internet connecitivity they mean NO INTERNET CONNECTIVITY. period, end of discussion. To that end you ask how do they service the very few machines inside. They are clean-room installed after cleanroon low-level disk reformatting and chip source verification (they rest I cannot say). Let sit, virus scanned by just about all the products avaliable, warehoused and then put into suspension to see if it does anything funny to other machines and then put (reluctlantly) into production. and at no time connected to the internet. The networks that are implemented are in seperate piped cable trays (that are now fibre to prevent taps) and all tray are visually checked and swept on a random schedule. The secure locations have even more procedures.....
To help you out there are some good installer generator products that pop up on giveawayoftheday that can help you out if you dont have time. There are also products (even from the old days) that allow the generation of registrations code for a speciific CPU or CPU/op system that you can use to support the offline customer.
I have (had) a big market for taking old PABXs out and replacing them with computer based PABX's supporting their analogue systems. A new PABX entry is R40,000 with little feature and flexibility. Voice recording is a very dificult add on at R150,000+ and I believe outbound only. I was very interested in taking advantage of this opertunity. But as you company problems and directive these offline and secure systems need not be considered so.........i need to get busy and do some other engineering.