Only someone familiar with that site (or someone going to the site and doing a physical survey) can answer your question.
I have seen small business hookups that used individual POTS lines connected to a local PBX box, and I have also seen individual POTS lines that were controlled by a virtual PBX in the telco infrastructure.
It is also possible that the existing circuits are not even POTS (such as ISDN BRI circuits).
How you would hookup multiple Vonage lines to replace the existing service would depend entirely on what the customer currently has, and what they want to continue to use. For example, if they currently have four incoming lines (POTS or ISDN) that feed a local PBX type box that connects to digital phones, you would either need to feed the FXS outputs from multiple Vonage ATAs into the analog inbound ports on their PBX box, or replace all of their digital phones with analog phones (and possibly you might also need to redo some in-house telco wiring).
You are going to have to visit the site and find out what they have. There is no way anyone on this forum is going to know what is at that site.--
We can never have enough of nature.
We need to witness our own limits transgressed, and some life pasturing freely where we never wander.