[Serusers] Trunking between two sip networks

Hi Folks, I am not a SER user yet, but hopefully soon. I have a need to interconnect two SIP networks just like we use a SIP to PSTN gateway (to connect SIP network and PSTN). Let me explain this senario in simple terms. Lets say I have 1 port FXO g/w. Then all my clienst in the ip network can use only one PSTN line to get to PSTN. And PSTN always thinks that the calls are originated from the directory number of this PSTN line. Similarly lets say I have 1 phone number, 1234(a)iptel.org ( one account from iptel). And I have my own network of Phones in a private SIP network. I want my clients to be able to talk to IPtel network just as the above senario, where IPtel would think the call is originated from 1234(a)iptel.org. True only one call at a time, just like PSTN senario. How could this be done ? BTW : This is not to make my phones in private network to have the ability to talk to IPtel phones. I can do that with my next-hop proxy setting and ip-dialplan. Cheers Sathya