First of all... What is ISDN?
ISDN refers to Integrated Services Digital Network that is a set of communication standards for simultaneous digital transmission of voice, video, data, and other network services over the traditional circuits of the PSTN (Public Switched Telephone Network). As ISDN is able to deliver at least 2 simultaneous connections, multiple devices can be attached to the line, and used as needed. That's why this phone line connection is still favored among companies.
What is the main difference between an ISDN card and an USB device?
If you want to create an ISDN phone line connection, firstly you need to connect your corporate PBX (Private Branch eXchange) to an ISDN line. (On the basis of my personal experiences, I recommend you to use Ozeki Phone System XE.) For building connection, you need to buy an ISDN phone subscription from your mobile service provider; in addition, an ISDN CAPI device is also required. A CAPI device is able to connect your corporate communication system to the public telephone network through the ISDN phone line. Although their features are very similar, the main difference between a PCI (Peripheral Component Interconnect) and an USB CAPI device is their physical appearance. While you need to insert an ISDN Card into your PC if you use a PCI device, you need to attach the USB device by using a USB cable.
Why is it worth to use an USB-based ISDN CAPI device?
It is easier and faster to install an USB device than a PCI card, since there is no need for integrating the device into your PC. This way, taking apart your computer is not required. You only need to plug a USB cable to the proper connector. This task can be easily carried out by anyone at the company, it does not require special IT skills.
This kind of separateness can be really useful in case of any malfunctions. As the device has not been built in to your PC, it is relatively simple to eliminate it by pulling out the USB cable from the computer. You can replace it easily with a new one.
In addition, its mobility can be absolutely beneficial. You can place it almost anywhere your ISDN phone line arrives. If your ISDN cable is not long enough, the device can be placed close to the telephone wall socket.
If you use a USB device, you can attach another PCI card to your PC (such as network card, sound card, modem or any other extra ports). And vice versa. Do not despair if it is impossible to place a new PCI card to your computer. Just use an USB device. This way, much more devices can be attached to your PC.
Almost every ISDN CAPI manufacturers offer at least one USB device, so this type of CAPI devices is available all over the world. It is easy to buy one from the local distributor or you can order a device from the producer's webshop.