However, there is a potential pitfall in this solution. This is because of the restriction that only one filter can be applied to each setup message.

A fax device, for example, may be configured to accept calls whose bearer capability is specifically Fax Group 3. However, fax messages which originate within an analogue network do not come with this very precise call type information. They are simply classified by ISDN as "generic analogue", just like a voice call.

Therefore, the fax device, which is looking specifically for fax calls, will reject valid fax calls from fax machines connected to the PSTN.

Similarly, if the fax machine is configured to accept "generic analogue" calls, then a fax message which originates directly from an ISDN network, and therefore may have the specific "fax call" type, will be ignored.