 Now, we are going to look at the last CSEF known as the interrogating CSEF. When we say interrogating, it actually means it can be used to query or interrogate the location of the serving CSEF of different IMSs if multiple IMSs are interacting with each other. To put in simple words, if there are different administrative domains belonging to different telecom companies each having its own IMS, in case certain serving CSEF has to be discovered because the user equipment has to interact with it, the interrogating CSEF has a role to play. So, the interrogating CSEF can subsequently forward or direct the SIP messages to appropriate nodes in the visited IMS. Let's look at an example. A session is originated by a user equipment which is visiting a network and accesses the IMS through the proxy CSEF. So, the first thing is done is through the DNS service, the IP address of the interrogating CSEF is obtained and then this interrogating CSEF is approached. It means the SIP messages are first delivered to the ICSEF. Then the ICSEF forwards or routes these SIP messages to the home network serving CSEF before any interaction can take place in the visited network to get certain services. It is done because ICSEF helps the overall IMS architecture to enforce home network control that is the overall control for admitting a certain user into a foreign network has to be done after taking a consent from the home network. So, the serving CSEF and HSS nodes are consulted that is why in short we say that all SIP messages received by ICSEF are forwarded to serving CSEF the user details because the user is registered there are tallied its profile is consulted and then a reply is made back to the visited IMS to deal with the user either to grant it a service or to deny it certain service. The routing function which is necessary for the interrogating CSEF to reach the serving CSEF of the user equipment that is the original serving CSEF in the home network has to be done it means how this routing process is going to take place the addressing information is obtained from the HSS of the parent network of the user equipment since it involves a lot of interaction with other CSEF the interrogating CSEF is a good place to keep track of the billing and charging because when multiple service providers are providing services together for their respective users the inter IMS calls and services have to be built differently from the user equipment called party and calling party within the same network in simple words the billing information is managed by the interrogating CSEF to make sure that whatever transactions or interactions take place between multiple IMSs have to be correspondingly charged from the respective IMSs here you see that the interrogating CSEF is very important because not only that it is used for forwarding the SIP requests coming in from multiple IMSs to their original CSEFs it is also used for billing and charging.