Use dialer profiles and multilink PPP to do the job. There is an example configuration (for a 36xx) on my web site -- see Listings 5-6 & 5-7 on the book pages of my web site, and read chapters 4 and 5 of my book for all the gory details :-) That example is overkill (it shows how to combine multiple PRIs for even more bandwidth), all you need is one PRI and one dialer. But it will show you the fundamentals of what needs to be configured to put dial backup over multiple ISDN calls on a single PRI.
Be aware that you have some learning to do. Make sure you understand how dial backup works before you go into production! It is very easy to create or copy a dial backup solution which does not provide backup under a wide range of failure modes...
Good luck and have fun!
Vincent C Jones
www.networkingunlimited.com
