It all depends on in which country you want to use your TA101 and what kind of ISDN you have there. The TA101 provides an "S/T" interface and requires an external network interface device (NT-1) for use in North America. Elsewhere, the NT-1 is provided by the phone company, and an external NT-1 is not required.

If your phone company provides and "S/T" interface then you do not need an NT-1, if your phone company provides an "U" interface then you will need an NT-1. Luckily these are not at all as expensive as a TA - you will probably be able to find some low cost pre-owned NT-1s on the Internet.

Best regards,

Edwin.