One of the simplest, clearest cases for VOIP Ive ever heard was made in the retail environment and had to do with distributing the customer-service workload among physically separated personnel. Its a case study Ive written up many times, featuring many different vendors. It goes like this:
A chain of stores/restaurants/beauty salons is spread across part of a state. Each has its own two analog phone lines, or perhaps a small, six-line key system; each has its own listing in its own local Yellow Pages, its own voice-mail service or answering machine, some customer-facing employees and some back-room personnel.
In the "before" picture, we see a potential customer calling the store that happens to be closest. We see an overwhelmed counterman too busy serving customers to answer the phone. The caller gets a busy signal or an offer to leave voice mail, hangs up and dials the next store/restaurant/salon listed in the Yellow Pages. Her business is lost.
In the "after VOIP" picture, we see the caller dialing that stores same published number. This time, if no one picks up on the third ring, a greeting is played that asks the caller if they want sales, directions or hours of operation, and please press 1, 2 or 3.
The last two choices offload a good percentage of calls all by themselves. The third (sales) choice rings every phone in the chain. (It also could be set up to ring the next available line.)
Whoever picks up can see which line the call is coming in on, and therefore which store has been called; customers never need to know whether branch A or branch B is helping them.
In the "after VOIP" picture, each branch store is hooked up with IP phones to DSL Internet connections. These phones act just like typical digital PBX extensions, but they exchange their signals and their voice streams over DSL to the IP PBX at the central branch.
These manufacturers typically have prior, or perhaps concurrent, product lines in traditional digital phone systems. They try to preserve all of the features of these legacy systems when they revise them for IP transport.
Such features may even include intercom, so that staff can back-channel through the phones to visually confirm that a part is in stock across town, during the customer call. Ive also written variations on this theme that involve bar-code scanning wireless phones, which verify inventory on the same session as an IP call.