CISCO : Bandwidth on demand

I have a CISCO router 1720 with 1 ethernet, 1 serial Leased Line (64K) to internet and 1 ISDN module (max 2x64K).
What do I want to do -> when my serial interface is on 60% of the bandwidth, the ISDN must connect to the internet and help the serialas long as needed until the bandwidth of the serial is 10% then the ISDN must disconnect, how can I do it. what commands do I need, give me a example of a program so I can try it out.

Thanks
Dirk