The imminent demise of ISDN has been talked about for some time. There now appears to be a date attached which makes it semi-sort of official. As of May 18, 2013, Verizon will no longer accept orders for new ISDN lines. They will also not make any changes to existing lines and will start charging more for the service.
Taking the place of ISDN will be a variety of Ethernet/IP-based audio transmission methods. As technology evolves, this makes sense. The quality of ISDN and the bidirectional nature was a vast improvement over the old system 5/7/10/15 KHz point to point analog lines. The one downside, ISDN equipment was expensive and the service was expensive to install and operate.
High-speed internet is available in almost every business and venue. Many times, there is no cost to access it and equipment is relatively inexpensive. Depending on the equipment, CODEC, and speed, it can sound almost as good as ISDN. For those opposed to using the public network due to reliability issues, there is always frame relay.
Time moves on, so buy your IP CODECS now.