What model/version and firmware IPO?
Although analog "works" with an IPO, it's not great. You will likely get echo, you will always loose at least the first incoming ring while the IPO waits for caller ID info, etc.
If your customer's system supports 4.0 firmware and has some VCM resources, I would recommend going with a SIP provider. Then you don't have the digital (Vonage Service) to analog (Vonage Adapter) back to Digital (IPO Box) conversions going on.