Our network currenlty connects to the internet via a 512 fractional T1 through a Cisco 1720 router. We also have 3 isdn lines in house that are used exclusively for video-conferencing. The ISDN lines connect to an ADTRAN NT1 (4 ports) which then connects to a Cisco IPVC video gateway. I want to know if there is a way (I'm sure some add'l hardware will be required) to use the ISDN lines (or a single line at least) as a backup internet connection when the T1 is down (unfortunately this has happened twice now in the past 3 months...). As always any and all help from the EE community is greatly appreciated.