Hello,
the dialer idle-timeout is based on interesting outbound traffic by default. With the value set to 300, the line would disconnect when there is no outbound interesting traffic after 300 seconds. Most likely, your interesting traffic is sent in an interval of 60 seconds or greater, that is why the line does disconnect if you set the dialer idle-timeout to less than those 60 seconds.
If you want the ISDN line to disconnect after 60 seconds, regardless of interesting traffic, you could use the interface command ´timeout absolute 1´:
interface BRI0
timeout absolute x (where x is the timeout value measured in minutes)
HTH,
GP