The phone number can be found on this page.

I also believe that craz might be right w/r/t using the value of the ticket. Once travel has started you might not be able to change the routing on the ticket, leaving that "extra" value sacrificed. Otherwise you could constantly book r/t tickets, fly one way and then use the residual value to book another "r/t" ticket. Even with the change fees you'd likely end up paying way less than buying all one way flights.