Hi -

You could use either -  the advantage of an IP network link, like broadband, or even a wireless private connection, is that you don't pay call charges.  With ISDN you will pay for the time you are using the connection.

We actually have a brand new device designed to work over IP networks, and suitable for exactly this type of application.  AudioTX STL-IP is a single box that you connect your audio into and it can send and receive audio live over any network connection -  including internet, private LAN/WAN, wireless, satellite, leased line, T1/E1 etc.  See Multimedia File Viewing and Clickable Links are available for Registered Members only!!  You need to or for more details on this.

Best.

Mo