c2821 ISDN site dialing into c3845 , call lasts only 28 sec

Well we have a remote site with 1 ISDN BRI on c2821
We have a 1 PRI at the central site on c3845
The remote site is supposed to dial in to the central site when the primary link fails
We have a Dialer-watch configuration done on the remote site & PPP callback request. The IP address is negotiated during PPP NCP/IPCP.

At the central site we have a PPP callback accept configured & the IP address is assigned from a static ISDN pool

Now the problem:-
The Remote site connects successfully into the Central Site. However the call lasts only 28 sec.

So what we did to resolve it.
We had four BRI S/T ports on the c2821. However only one was connected to a BRI line. But at all times all four ports were part of the rotary group.
This was causing the dialer interface to cycle between the 4 BRI S/T ports.
So we removed the other 3 non connected ports from the rotary group & voila the cal lasted till the idle timeout.