setting up access to intranet - help

Hi
I have developed a company intranet, set up IIS and users within the same building can now log on to the intranet, however, I have now been asked how users from other offices will be able to log on, I assume if they can dial in to the network they will be able to access the network no problem, however the next question I am then asked is what about users on customers premises, how will they access the intranet?

Can anyone advise me as to how to set all this up, I assume that maybe the best way is to get the intranet hosted elsewhere so that others at client sites can log into it, as I cannot see how they would be able to log into the network as this would be a major security issue. Any ideas and guidance would be greatly appreciated. Thanks

Yvonne