I've translated the remainder of the article about SIP Broker. In the next paragraph, the author has explained why 7777 was not available.
I haven't found an email address for the author, but he gives his SIP Broker number in the article, and you could probably look him up at his "ipvision" company's website.
IP VISION
The author's company is also on the list of members of "ENUM Trial Japan".
edit:
I've sent Tony an email address by PM.