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To the uninitiated, it may come as a surprise to learn that an ISDN modem can connect each of its two 64K data channels (B-channels) to an ISP's 56K V.90 modem and get something that acts like a single 128K connection.
The tricky part is that the router that you connect to must somehow know that both of these incoming connections are really supposed to be grouped into one combined connection. In order to do this the ISP's router must support a protocol called "Multilink PPP" (dialing in with your second line is sometimes referred to as "joining the MP bundle" in router logs). Of course, your TA or router must be configured properly to use both channels as well.
Some ISPs explicitly support ISDN and Multilink PPP while others only claim to offer normal 56K dialup but some of their phone numbers actually do work with ISDN. Be aware that using ISDN with an ISP that doesn't officially support it may be in technical violation of the TOS (Terms of Service) agreement and your clever hookup could stop working at any time. That being said, many people have had good success with these ISPs.
Caveat: The information in the following table is subjective and may become out of date. ISPs without explicit ISDN support may or may not work for you even if they have worked for others.