Jake,
As Jim mentioned you need to set up a VPN. I have done this many (almost too many) times. If you have a VPN/Firewall device already in use at your main office you should have no problem setting up a VPN on it. Once that is done you need a hardware device (VPN/Router) for the remote locations. Our favorite device is the Linksys BEFSX41 (lovingly know by us as the "Beef Sex 41".
The BEFSX41 will allow you to set up a VPN to your office. The BEFSX41 will use the dynamically assigned IP address that your cable modem gives it. Then in the VPN settings you have to give your computer and IP Phone an internal IP address which should be assigned by the BEFSX41 via DHCP. The VPN will also know the internal network address of the telephone network (i.e. 10.0.0.x ) or whatever internal IP address range you are using. If you need help setting this up please check out the following website -
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- they have excellent information on how to make this work with several different devices.
If you need further help I have some documentation that I have prepared which may help you somewhat.
Good Luck!