Any device that uses a modem will, at best, give hit-or-miss results when used with any VoIP device and service. I send a few 1 or 2 page faxes a month over my OBi110, and it probably works roughly 80% of the time on the first try. But there have been days I've had to try at least a half dozen times before it succeeded. It's not important enough or frequent enough to warrant me getting a POTS line for this purpose, but for POS, it would be very frustrating for both the merchant and the customer to retry every transaction that doesn't go through. It could be pretty embarrassing for the merchant.