Checked the IP pool in BRAS, found ok .
In this network by two way we are providing the IP to the customer one dynamic way another is Static way.In the case of static IP we are providing the IP through iTellin.
Then we have checked the IP pool configured in iTellin .Found some IP is binded to the user name which is belongs to dynamic IP pool. If these ip has been allocated to other users, ip conflict may be occure when user get the same ip address from iTellin.
Removed those ips from the dynamic IP pool then problem is solved