When a device receives a SETUP message, it can apply a filter to that message. This filter examines the information that arrives in the SETUP message so that it may process or ignore the message. Only messages which pass the filter are acted upon.

In practice, equipment is only able to apply one test to each parameter. The kind of information which can be filtered in a SETUP message includes:

• Called party number (destination number)
• Calling party number (origin number)
• Call type (Bearer Capabilities)

ISDN calls can be divided into two main categories: analogue data and unstructured data. A SETUP must include the call type (bearer capabilities) of the call. Without this information, the SETUP is not valid and the network will instantly disconnect any such call request, citing "mandatory information message missing" as the cause. When a SETUP is sent by the network to the destination, the call type is also mandatory.