A company I worked for had two sites - one in Scotland and one in England. When they started up, they installed Microsoft Small Business Server in the Scotland office and joined the two sites with ISDN lines, with the Scottish end being the domain controller. A couple of months later (all of this before I got the job of IT manager) they saw the ISDN bills and the accountant hit the roof! To cut down the bill they disconnected the routers from the ISDN, meaning that the English end was cut off from the domain. This meant that nobody could add new users, disable the accounts of people who had left, change passwords etc, and the server logs were filling up with error messages, seriously impinging on disk space.
After I got there we got ADSL lines in and set up a proper VPN between the sites, giving all users internet access at reasonable speeds and making system maintenance so much easier, especially as I was based in the English office.
The ISDN lines were just used for credit card transactions after that, until we could bin the contract.