I am on the board of a volunteer association (Internet Society, Dutch chapter) and it would be cool if board members can call each other without having to declare the phone costs periodically to the Treasurer (too much hassle).
The Treasurer buys a certain calling card at the best possible rate he
can find. He takes all the telephone numbers of all the board members and uploads them in some way to the telco selling the card.
Only people whom's numbers the Treasurer enters periodically can call each other (caller has to have callerID switched on). The numbers of the other board members are a certain 0800 number with a single digit added for the person you want to call.
There might even be a voice response menu with the names of the people you can call. Useful for reaching other volunteers inside the association. I can never keep track of their numbers, often new ones every month. (They are usually 'inbetween jobs' and use the Internet Society to find a new employer by doing some volunteer work.)
Also a great way to map the communication patterns between group members. Some hate the telephone and prefer e-mail, others only _read_ e-mail. Others use everything to communicate but should stick to e-mail to save other people's time...