No setting your agents to call in on dialup will not do anything. The simple answer is all agents must be able to talk to ePO at all times if you want that kind of visibility. This means either putting your ePO server in the DMZ so agents can reach it from the Internet or put an agent handler in your DMZ which would allow your agents to always speak to ePO.
Thanks for the reply and for the white paper. I will look into that option.