dialicious.com is probably what you are looking for. You can access their modem via any computer or mobile device in order to dial into your systems from wherever you are. I've used them in the past in an emergency (I was at the airport and needed to fix a system that was down) and it worked fine.
If you have the cash, I would suggest getting your own dial-up server and making it accessible via the Internet. You can get a low end Cisco router and place a modem in it and go from there. Estimated cost $2K.