I've done the DNS settings on my internal DNS. Configured users accounts to use IM server. Simply its working perfectly within the domain users. I've tested its IM, File transfer & voice features without any problem.
Actually we are more interested in the voice feature using our own IM server. I want to do the same setup in our head office. Currently we have ISA on both offices. We are using exchange server of head office for our mails. (using POP)
Tell me how can I publish IM server, so that we can access the IM service on exchange installed in our head office from here using MSN Messenger for Exchange. Does it require Gatekeeper etc.???
IM uses http to transfer messages. However, I'm not sure how the Exchange IM client resolves servers or other clients (whether it's through DNS or a modified implementation of ILS). You shouldn't need a Gatekeeper, since this is used for Netmeeting only AFAIK.
Have you thought of using a VPN between the two offices? This would remove ISA from the problem, and you'd have private LAN issues only....