It is important to find out if your ISDN adapter supports this and which protocol or protocols your ISDN provider supports. Some have both protocols and at least one currently available use its own proprietary protocol. You will want to find out what protocols they support and what ISDN adapters they prefer and why. The why is a big issue as many of the ISDN providers are also selling you the hardware so make sure you getting th e hardware and service options you require. There are other issues that a user may want to deal with when selecting an ISDN adapter.
Price is an obvious issue but the setup the of adapter is equally important.
The robustness of the setup software is an important factor. These devices are not as simple as analog modems to setup. Once the setup is complete however the benefits will far out weigh any initial frustration that you may have.