Hi srinivas,
for the first issue, you have to put your voice traffic in priority queue. You could use something like this:
class-map match-any VOIP
match ip dscp 40 (or 46, or what you want)
match ip address (voip traffic from a specific subnet, or host)
!
policy-map PRIMUS
class VOIP
priority
police 1000000 conform-action transmit exceed-action drop
class DATA
bandwith remaining percent 90
!
interface X
description ** PRIMUS **
service-policy output PRIMUS
!
Then, for ISP failover, and possibly loadbalancing (but not if you have a full BGP table, of course), please check:
http://www.cisco.com/go/oer
HTH
Andrea