Call relay is basically the same as H323 proxy. In it used to handle incoming VoIP calls from your customers and send them to your vendors using VoIP.
In this way:
– you have all the information about the calls which have been made
– you hide customers from vendors and vice versa
The call relay isnt a proxy gateway it actually deals with calls it also enables you to codec negotiate on a call by call basis as well as be a central location to collect cdr’s from.