ADSL as a Alternate route or backup route
 

Hi guys
I have a small bussiness firm with about 15 servers
The linx box is the firewall and gateway to these servers
Now we have a backup ADSL line which is suppose to be used as a backup option , incase the main T1 is down
We have links from 2 ISP

ISP 1 (Main link )--- 62.150.24.X
ISP 2 (ADSL) -- 62.215.189.Y
Both ISP have there different DNS servers

What my question is how can i use this ADSL line for my backup , incase if the main link goes down.
Do i have to change any Ip address in the servers .
I tried routing the 62.150.24.X network through the ADSL line , but it doesnt works
Pls guide me as how this can be done ?
We got 2 Web servers too

Awaiting reply
thank a lot in advance

bhagat panwar